Think of it this way. An overdrive trans does not change the speed the drive shaft at any given speed. But an axle gearing change does.
Most of the changes to rear gears lower the drive shaft speed if they change it at all and that is what I was saying.
I once bought a '50 Chevy 1-ton that had a 3.42 rear under it. The guy I bought it from said the old 216 would not provide enough torque to move the truck with any load at all. I never tested what he said but I believe it.
There is a reason for those original deep gears and I think some times people have unrealistic expectations about what they can accomplish with an overdrive or gear change.
