Some of the original brake drums are really big, and don't fit modern wheels as you found. This is why test fitting any wheel before buying is a good idea. This is a known problem with 19.5" wheels on the 1.5 and 2 ton trucks too, some fit but many don't.
I don't think you'll find any other 19.5" wheels that do fit, they are pretty standard.
However, the 14 bolt axle swap I think is the best solution and an improvement in gear choices, brakes, and wheels, so all around a better deal.
Running just the outers with no load in the bed should get you by until the axle swap.
The larger stud hole in the new wheels should be solvable with the proper lug nuts.
I assume you're using the clamp rings from the P30 too? Take one in to the parts place and ask for a 1/2" lug nut big enough to fit the chamfer.
Grigg
