I would guess that you could drill the hubs out larger to fit bigger studs. The front hubs separate from the brake drums once those are both off, and the rear hubs pull right out after taking both castle nuts off from the axles. So it should be easy to do on a drill press. You'd need studs with short heads on them.
The rim offsets on the old 350s differ greatly from front to rear though... maybe as much as 2 inches..?
Hmm, maybe not a spring project since I am running out of time before planting gets here. But since I do have a parts machine with hubs...I'll have to give it a try.
You are correct about the two inches. The Pioneer wheels will need 2-2.5 in the rear.