You have a lot of stuff going on here and perhaps some mis-understandings. There is a 4 speed overdrive transmission used in some later Chevy trucks and some Mopars. I can't remember the model but from what I have seen they are quite costly.
Many on this site are using T-5 OD transmissions and they work just fine. Lugnutz has a good write up on the care and feeding of T-5's. They can also be costly. Much of this is dependent on what your skills are and what you have to pay others to do.
The old tried and true BW OD trans from 55 up cars/trucks (SM319) is also a good option. It would of course require an open driveline and you would retain the column shifter. It's the closest thing to a bolt in.
Something you have to take into account is the brakes and steering. You can make just about anything go 70+ mph but you have to think about how you are going to stop it.
Fred
Last edited by rfs56trk; 01/03/2015 6:13 PM.
