thomas,
The transmission is more according to what you like. The three speed of course would be easier to install since you have the parts for it. My 1/2 ton had a four speed and I like it because it is sync in the top three gears. You would have to find a four speed from a half ton truck that would have the rear piece for the ball. My U-joint went bad on mine so I got a u-joint from a 3/4 ton. Had the rear part of the joint machined out and the rear part of a three speed joint welded in. Much more heavy duty than original joint.
The early 3/4 ton Chevy trucks also had enclosed driveshaft between transmission and carrier bearing. I'm not sure up to what year they were made that way though, at least I think till '51 or so. I think the transmission would also work from these trucks, but you would still have to come up with a u-joint. I hope this will help.
