I believe the only difference to the frame and cross members is bellhousing cross member which can be changed with some work. The floor and wiring harness are actually different if you want the truck to look original, though anyone thinking Powerglide isn't doing a serious resto.
Personally I'd rather retrofit a Hydra-Matic than an early Powerglide. But given a choice I'd go open drive and a modern automatic.
Hmmm, the one I'm currently working on I'm going from Hydra-Matic to TH400 with a V-8. That might reveal my bias a little.
