A local guy is using an S-10 4x4 rear end under his 50. You'll need to lose the stock bellhousing and go with one that accepts modern stick pattern, like 55 to 62 passenger also trucks.
Also the 3 speeds from the later cars/trucks should be fine with your stock shifter set up. I'd give Buffalo a call, has a zillion fixes for these early Chevs. What ever you want to do he has all the solutions.
