What year and model P are you talking here? Some outside company made those brake parts, so you may find elsewhere.
There's always parts, if you're willing to search and pay.
When a piece is stamped out, it's often difficult to repair to resto quality. But can usually be done, just takes a long time to fit, weld, and finish so it doesn't look like it's repaired. So the cost of parts is a big savings in time.

One more thing.
I don't see where the block was similarly checked for flatness. Take a machinists straight edge and shine light under and measure gaps with feeler gauges.
If that .007" "low spot" has a corresponding one on the block, you'll could have leakage if just one side is decked.
It's a tough call, because head is close to spec but one spot is suspect and overall damage from years of mating dissimilar metals. I'd make sure my machinist has a lot of experience with flat heads.
This is going to be more common as these motors age. The damned if you do, and damned if you don't scenario. It's expensive to cold spray something of this size all over to build up and once metal is removed, it's gone. Eventually, with no new parts available, cast will deteriorate to a point of unfit for service so it remains a scavenger exercise.
With the right care and anticorrosion agents in the coolant,
it should function for another 70+ years.

No one answered on the powder coating question, so if you do, use high temp powders, just for less possible discoloration over time.
I don't see a need with the modern engine paints, however. They are good to 500+F and seem to be easy to apply. I did the Cad with Eastwoods, only to get the color. Rattle cans are OK too for something like a head.
You can ceramic coat also, and preserve an unpainted cast iron look, but neither of the thermals were factory finishes. They came out spray painted at the factory. Any thermal coating is going to require an oven bake before to devolatilize and after to cure. Cracks can grow from thermal cycles. So consider you're needs before your options. You don't want to do all that head work only to get a crack after/from appearance coating.
ps: I'd opt for new studs. Cleaning rusty threads doesn't ensure integrity. If you snap one during torque, it's start over.

I'm of the "why bother" position, unless local smog rules absolutely require. These cars are basically exempt as built.
(Some places may require vintage plates for that) If it needs rebuild, a PCV won't help. If you want a car that came with a factory PCV, buy one.

To get back to stock, about all you can do at this point is try to locate a stock radiator and see if solves the problem. Then take up with the radiator shop, if it's not right. Like I said, the air side of a radiator controls it's heat transfer.

Be very careful handling concentrated sulfuric acid. Need neoprene gloves and splash/eye protection.

Adding conc. acid to dilute is OK. The other way is dangerous and will cause uncontrolled heating and boiling.
The old chemist saying - add acid to water, not water to acid.
Probably best to buy or make up a batch at the correct concentration and add back to refill the battery to level. From the recommended specific gravity, the correct concentration can be determined. Prolonged use with insufficient acid will cause it to fail prematurely/not deliver the correct ampacity.

Or simply go buy a new battery, now or when it dies. Just depends how much you want to fool around to save a few dollars. Garages used to do this, but today, I don't anyone handling battery acid routinely. They just sell new batteries. They used to sell hydrometers to test the acid, but I haven't seen those around much either. I think mine died of hard-rubberitis a long time ago.

In your external fan picture, the front shroud must have been taken off. Do you use it when it's not fan assisted or is it missing? It directs the flow to the radiator from below. It's the ribbed pan shown in this photo.

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I was looking at the Auburn auction catalog and there's a slew of 6 figure classic prewar Packards, and one comparably nice '50 "8" 4 dr, and it's listed for estimate, $8-12K. Even used Volkswagons are commanding much better(2-3X) prices than bath tubs... I wonder how they sold any at all back in the day, strictly off brand loyalty and pre-war reputation, I guess.

http://www.jessersclassickeys.com/

There's always, Joey!