All the trannys you listed will not improve your speed as high gear is 1:1 on all of them. Plus you are talking about major surgery to even fit a Saginaw to your torque tube. Why not just make the leap now to open driveline, makes life so much nicer. With the open drive line then you can think about great things like a T-5 overdrive trans. If you must stay with torque tube then just swap in Patrick's gears.


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