I've used the Saginaw 4 speed and it's fine but better set up and possibly much less money would be the T-5. Has an overdrive gear, you don't need to buy a shifter and being a top loader it's centered in the floor. Need to lose the torque tube and go to more modern driveshaft and replace or modify the rear end.

Lots of how-to articles on the T-5 swap.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed