It's out.
What? Read the directions? U got me! I read that section last year. I'll reread now and see what I did wrong.
I'd recommend:
Sling front and choke bell housing, not at the trans mount.(it wants to pivot not lift in back)
Center pull just before #6 cylinder (it took a while adjusting straps for COG)
pull #8 plug
remove the static clutch linkage so the clutch paddle clears
Take the upper trans mount off, I didn't, and it complicated life(lost ~hour).
Make sure your motor toter clears over and fits inside your lift, I had to rework mine. Out smarted myself trying to add stability too wide.
ps: easier to store with trans on it. less work to dismantle.

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If the ammeter is discharging then not, somethings pulsing, like the turn signal. With key off the charging system is cut off, so when it's on, it's pulling current//discahrging. Apparently a short other corrosion related issue might be there or faulty signal flasher. Check the dial frequency and see if it's about the turn signal time span. If not, it's something else.
I'm not a fan of screwing around with old wiring. The diagrams are on here, it's not that big a job to rewire the critical stuff, at least you know it'll work.
Test the horns first. Pull the lead wire off and hot jump each one. If they growl, they're probably OK, just clean them up. If no sound, they're shot. Good place for critters to hide stuff or water to sit. Then work back to the relay. The horn relays are readily available so I wouldn't screw around if it's rusty. I'll donate the one I'm not going to need, if necessary.

I couldn't find any good safe places to bolt up the leveler, so I'm going with slings. One under and around the front mount yoke and one choking around the trans mount all to a big clevis. Should put lift point close to motor center. Front is off, so should be able to raise up over the cross member, push car out, and drop on my new "D. Grubbs wooden motor toter". Tues is golf, so won't do til about Wed/Th. Let me know if this sounds squirrely, what ot watch, or you have a better approach. Tks.

A few words on electrical safety:
Anytime a preventive safety device like a fuse blows or circuit breaker trips, it's due to reaction to a fault condition. In this case, overamping. It is an indicator and warning that a non-normal condition (fault) caused it. Good practice is the fault(s) needs to be found and corrected before re-energizing the circuit.
Simply replacing a blown fuse, resetting, or bypassing, in a faulty circuit has consequences, like experienced, and further damage or injury can/will occur. That's what the fuse is there for, to protect the down line equipment from overload.
Always check the circuit and correct the fault before you re-install a fuse or reset a breaker to make sure it's not going to just blow/trip again and cause further damage. (You're life may depend on it)

As good as this forum is a source of info and suggestions what/where to look for, it's impractical for diagnosis.

Since it's an SAE fitting, that's a good place to get the answer.

Point of Safety - check everything before you energize any circuits. Adding larger supply cables to an unknown issue invites bigger consequences.
Yeah, it's low voltage but high amperage and that's what kills you / does damage. Popping fuses and sparking is not considered good electrical troubleshooting and in some electrical classifications will cause major harm.
First check everything is properly connected and in good condition per wiring diagram. With a meter, check for continuity and amp draw. This may require removal and sending to a shop if not equipped. After all the circuits have been "rung out" then energize the system. You can disconnect non essentials just make sure they're taped off / out of harm.
Before you continue, I'd check the voltage regulator(VR) and generator circuit.
The lights are on a separate circuit and don't directly impact the starter circuit.
What I'm thinking is the VR senses the voltage drop from the starter draw and closes, as normal, but opens to a fault. Either the generator, wires, or the VR could contain the fault. This was held out in some systems until the motor started, but I don't recall seeing that in the Packard wiring diagram.

I think it's been generally recognized for a long time that around 60-70% of the braking occurs with the front brakes/tires. So if they aren't good, braking suffers regardless of type.
If you compare the design of the friction surfaces drum vs disc pads, because the drum pads anchor/pivot at one point, the drums have variable contact area as they wear while functioning disc pads wear even/stay the same. The disc clamping force is more uniform doesn't pivot or point source like in the drums. We all know discs can stick and not work good either and still need regular checks especially if you own a fresh water boat trailer with them and frequent brackish water. (dah!) And rotors warp too.
Even having self adjusters would be a big improvement for the early drums.
The economics and conditions of operating, now vs then, has shifted alot of the mechanical needs. I can remember the brakes were bad on my aunt's Pontiac, so she just down shifted to first and slowed to a crawl and told me to jump out the door as she left me home. Thankful, I lived on a flat street and not a hill. Of course, what's a young boy not going to do if told? I think that constitutes child abuse today???

Bet the EPA loves those! How do you spell - FOGGER?
Amazing how perception/"knowledge" changes over time. Smoke and particulate was a sign of industry. Black rain anyone? We can't even handle colorless odorless CO2, which we exhale?
They complained about controlling acid rain, now my pool's pH goes UP after a rain?????

There's a picture of the motor in the plate part of the 23 rd series manual(on this site) that shows the air cleaner. I think I even posted pix of mine after restoration last year for someone. It's an AC Delco dry type for the 2 bbl Carter carbs and is common to alot of early straight eights, so don't get sucked in for a "Packard" part. Packard didn't make air cleaners or alot of other common parts; brakes etc..
The wet type is kinda duck billed looking and extends across the motor.

If it's turning slow, check the starter and see if it's getting hot.
If the motors stuck, it'll load the starter and heat up. If the voltage/amperage is just low then the starter won't heat, just turn slow.
Make sure you let it cool between starts, dont; keep cranking if no start. Wires and starter when hot require more juice. Heat in wire will also indicate too small wire size or poor condition.
I don't think this has the carb starter but don't know - someone with later model.