There's a "parts cross-reference" tab on the left that has most of the available new service replacements listed. May not be 100% up to date but good place to start.

My standard recommendation is anything more than 30 years old should have the wiring thoroughly checked/replaced all or as needed. This era used the cloth wrap over rubber insulation and it cracks first then deteriorates flakes off, plus the exposed copper will corrode unseen, increase resistance and cause heat and lead to fires. I had a horn wire catch fire on the CAD, melted the relay, when the paper insulator on the horn rim degraded and slowly shorted. The old slow fuses won't blow with a drain like that. Lucky I caught in time. Many have lost valuable cars from trying to "maintain" factory originality. Don't be foolish, if your already seeing wiring issues.
My '49 wiring was pretty much stripped clean by field mice and insectia.

I think you have to adapt to modern maintenance standards and materials in the context of the old ways. What was recommended in 1950 probably doesn't exist today and certainly won't be cost effective. Even the fuels have change alot. Two camps; originalists and the smart. Boy that should get 'em going!

Prestone "flush" is plain old TSPP = tetra sodium pyrophosphate. Available at most hardware/bigbox stores. An alkaline cleaner that's good for steel, etc. Aways want to be alkaline with iron, and not too strong. More iron can dissolve at high pH ~13, than above 7. (Iron only, not good for aluminum!) There's a curve for that.
I'd just flush it not running with the hose in the cap and drain open until the water comes clear, then clean it with flush and running to heat it up. Drain, flush again, and drain it dry. Can blow some air through it to get the last and then add the 50/50 antifreeze. I use DI/distilled water to mix as the premix is more expensive, and tap water can put hardness, pH, which can lead to deposits in the system/breakdown of the EG. I did that on the '49 and then found the hose neck was rotted and had to be repaired. I tested the thermostat to make sure it was correct and opening. One pin hole and they won't work. Capsule has to be intact. It would idle for an hour with no change in T. It had sat in a field for at least 5 years and not ran for a long before.
Old radiators have a heavy solder content, so PbO layer shouldn't be disturbed with what you clean it with.
Back in that day, '28, alcohol was the antifreeze of choice, but it boiled out overtime and had to be replaced frequently.
The modern anticorrosion inhibitors in the EG antifreezes should be sufficient to protect what you have. Change at about 5 years.
It's hard to say what or if the "verge of overheat" is caused or from as designed. These radiators are not pressurized, so boiling will be about 212 or so at sea level. Since you're operating in high elevation,(Denver) that could be your problem. BP goes down with altitude. It's really not a problem until it comes gushing out and loss of fluid. If it boils, then you get steam pockets, and steam is not a good heat transfer medium and superheats, pressure rises in block, and then it expands and moves everything out. Not good for the lubricated side. Pressurization inhibits that nucleate boiling issue. {They weren't too heat transfer swavvy in '28.}
The air flow is more critical to remove the heat than the cooling side film, unless flow is retarded due to sludge, plugged passages, or weak H2O pump/if not a thermosyphon system. Check your belt and fan and any missing air dams. Impellor may have rusted over time. Something to check as is the thermostat for operation or blockage, also.
Fine surface roughness has little or no impact to HT area as it's within the laminar zone. The fins are what help. Plus not much air moving inside the engine compartment.
My personal rule is not to experiment/play around with hard to replace antiques and modern 'solutions' without exhaustive testing and verification. There all kinds of hearsay suggestions out there without proof, but sworn by. Not saying they aren't good, but without documented proof - won't stand up in court! One reason I usually don't recommend products and certainly qualified to do so.
Definitely don't run just RO/DI water without any inhibitor in the system, even for short runs. Water itself is corrosive. Old cast, gaskets, etc are of unknown condition, so you don't want to open up any faults that were sealed by corrosion products... When one operates a 'million ohm' water system, they use non-metallic pipes, etc. so it doesn't pick up any metal ions.

I suspect you drop the first three Packard subnumbers to reference the AC numbers.
I'll go down the garage and see what's what. I'm pretty sure I have a chart somewhere.

added:
If you look at file ending 75, that's the factory prefix number adds I mention above. Good thing to know when searching parts, how it was done.
The 9589 is the same as a 9294, so if you look across from the early P numbers to Olds on file 90, that's the equivalent as already noted.
The other file is all the rest of the years for P.
Kev, should post these somewhere on here or tell me how and where? Last time only he could do those things but I'm not uptodate.

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Did you look in the shop manual? I've haven't in some time, so don't quote me. I'm expecting some info maybe there and hidden in some sections, or if P published body manuals? P is kinda in the before period of a lot of automotive science and published info. And the co. records didn't survive all that well and what did got dispersed through the merger. You can maybe get some from the drawings through PAC or Studebaker museum. It was more a competitive trade secret world that everyone spied on/reverse engineered or got tech from vendors. Someone was supplying those rubber parts to P and would be a reference.

Need to respect the torque values also, to get proper body fitment. Torquing require clean and good threads. Compression is the reason. Different rubbers compress at different rates, aka durometer, to get the correct gap. So the rubber has to be correct type, formula too. But I'm bothering with technical details... Just as long as it doesn't shake, rattle, but rolls should be fine.
Making your own rubber parts is above and beyond, frozen or not!
fyi; If the body twists, then no amount of adjustment will work. There's usually measurements that are indictive, for collision work, etc. Not sure what P did for that?
I'm trying to forget my body mount "ordeal" on my truck. This is elegant.
Generally, the "frame off", means the two pieces were completely separated and refurbished then mated, not piece meal. It's kinda misnomer because the body comes off, not the frame? Should be a complete body off and frame restoration. But I'm messing with you now.

I'm reading an update bio of the founder of Cummins diesel. Synopsis snippet: In his early years circa turn of the 20th century, while working for Marmon, et al, he developed noted driving and machinist skills.
His family had ties to the local banker who owned a '07 Packard open touring car. Banker did not know how to drive it! He apparently bought it to have his wife and himself chauffeured around and she fired the driver for lighting up a cigarette while driving. Talk about "principled" Packard owners, open car, no smoking? Later, cars wouldn't sell without ash trays and lighters!! Vape cartridges ended that in lieu of cup and phone holders. The banker hires between jobs teenaged Cummins to drive for him. They had to take the Packard down off the blocks and remove all the coverings even for the tires. I guess that's where the perpetuated storage idea of putting them on blocks originates, for the old tubed gum rubber tires? They didn't drive in winter in southern IN both for the snow and poor road conditions. He loses his job at Marmon, etc. and eventually the banker puts him in business as a mechanic and machinist so long as he kept driving for him. A lot of the parts of the day were hand made one off, hence the need for machinist skill. OJT max. And we quibble about "parts"?
He ended up getting all the vehicle repair business for the whole county, as word spread of his prowess, which amounted to less the two dozen vehicles! Today, I'm sure there's more on my dead end street than that. No repair shop could survive on that volume today.
Being on call to drive the guy down the street in his Packard and not having uber, GPS, etc.? Those were the daze. All no more than 125 years ago. We call it progress, I'm not so sure about that. Some yes, some no.

Like the old directions joke; go past the red house painted blue and turn left at the intersection. They switched colors on the body filler cream hardener across the board some time ago, and I didn't know of it. I don't use much filler, I like metal. I use more MEKP, clear stuff, as I do gelcoat work on the boat, fixing chips, etc. I bought some last year.
I had a few small weld imperfections to cover on an inner fender repair, so I got the old can out, no hardener. All fresh stuff is in the freezer and didn't want to wait on thaw.(BP loses 1% a month at room temp, prevent the filler from setting up) I didn't need much, so I go to the closest parts store down the street and all they got is blue. House brand filler too. I checked across the street, only more blue. I went to NAPA and all they had was blue. I see a pattern here! I bought blue. Conflicting store stories on whether it was the same. You can still get red, but they don't stock it. The chemist in me, decided to check, and it's the right peroxide/CAS. I used unknown tons of this peroxide, installed onsite storage, and we even made it at other locations. Looks like they did change the plasticizer though to ECHA. I suspect it's cheaper to get from China that way. It hardened, so no issue. Nothing worse than having to scrape off gooey sticky filler that didn't set up.
The internet "why" story is red bled through to top coat, but I've been using filler for nearly 60 years and never had any issue with bleed through. That's what primer is for. Could be artifact of water borne urethanes, but I don't use those, but doubt the story. More likely some body man messed up.
Plus it turned turquoise green, not blue, when I mixed it, so go figure? Maybe we'll have green next?

You have to unbolt and raise the body off the frame a tad to change the cushions. ie, take the weight off and space to replace with new ones. If the bolts are rusty, it can be a chore if they don't want to separate. And maybe a two man job, one holding wrench inside, one under. Get them all on with bolt in, lower, and then tighten it back up.
Surprised the outriggers are bad, but the body isn't corroded through as well? Thicker GA steel on the brace. Maybe another surprise waiting for you. And didn't you just have the carpet out?

I just replaced all them on my P/U truck and could not use impact they way they were arranged, inaccessible floating cage nuts. I didn't know they had an issue with the urethane and the "baja" construction zone ate them alive and other suspension parts - long story.
So you'll need some sort of lift or jack and way to reinforce the body/distribute the load. Usually, you can get them one by one on the outriggers, but since you're changing braces, it's a bit more complex. Times like these one wishes they have a lift or body twirler and take the whole thing up.
I couldn't get my twirler under the truck without a major mod to it. Time you get done fixing all the gremlins, you'll have a complete resto.

If it gets below 32F where you live, you may want to reconsider permanent installation.

Anytime one introduces water into combustion it lowers the combustion temperature by making steam which partially offsets the loss of exhaust gas thermal expansion to produce power.
It's an old proven heat balance concept for burning higher BTU fuels so the temperature isn't as hot. We used it to reduce NOx in our HW incinerator. Diesels now come with DEF injection, which is basically the same principle for NOx reduction, only they add urea to prevent the water from freezing.

Related:
I saw Biden EPA is going to try to approve E15 year round, which they were doing on the sly in summer/low fuel supply periods and not telling. FI vehicles don't experience the issues of ETOH fuels with carbs. Some FI vehicles can't use E15, so check around if it changes where you buy gas this year. The corn farmers are happy, but the environment suffers a tad, so pure poly-tics. Right now there's some refining outages driving gas prices, but crude supply is up, so prices should come down later. I don't expect them to be where we like them, crude is still above $80.

HH is right should be glass and "white", not yellow. Gasket in there too. All that era were still glass. I went through this but don't recall, I think I got one off internet somewhere. I'd have to research and see if there's one here for reference, if you don't find any.

I got turn NOS front signal fixtures from MM but suspect they'd all be gone by now. I'm sure some hoarder has them if they'll let go. That's half the battle in this sports, the "search". Parts unfortunately were never trained to come and find us. Sowing many seeds is a good choice. Leads come and go though.

The shop manual should also have the complete wiring diagram and that will have the wire GA and insulation color. I know I got it, not sure in where.
Rules of thumb,
The connecting stud is usually sized for the proper terminal connector. If hole is too big or too small to fit, it's the wrong wire GA. Terminals come with a 2 size range typically, like 14-16, 18-20, etc. By good quality and stay away from the cheapo offshore stuff. International EC permits stuff that's "iffy".
6 VDC is typically larger GA due to increased amperage due to lower voltage. V=IR, R constant. Diam determines ampacity so non-linear, R^2 law.

Make sure you got a good crimper with the proper dies. Terminals can be obtained from the usual auto electric supplies or carriers. Del City, etc. Even though its nor OEM, I like to shrink wrap each connector. Gives a finished touch and prevents corrosion and puking apart. SAE doesn't permit soldering, but I don't see an issue if you want a better connection, however, residual acid solder flux should be removed. I only do it, if it's a single crimp w/o the insulation crimp. Plus the insulation may discolor and why it helps to shrink wrap.