There is information "out there" on swapping a torque tube rear end from something like a '48-54 PowerGlide equipped car. From what I remember, it is a pretty straight swap. I might have a text file with the instructions somewhere. That will allow you to keep the torque tube but have a 3.55:1, and stay away from the installation difficulties of swapping the ring and pinion only.
The Powerglide arrived in 1950. It had a 3.55 rear end and 5 wheel-bolt axles. I recall a Tech Article on a Powerglide conversion being "out there" but cannot quickly remember where (Jim Carter, maybe).
The 3.55 ring&pinion swap suggested by SWEET is the closest you will get to a "direct swap" change in the rear-end ratio in a 47-55 1/2 ton GM truck. Many of us with that conversion have no problem cruising all day at 60-65 mph (with a later 235/261).
What size/year engine do you have/want? A 1954-63 235/261 would be the closest to a "direct swap" engine upgrade.
