The 5.14 rear gears were an option with 4.57 being standard in a 3/4-ton. You might find a 4.11 gear set from a later truck with the Eaton rear that would swap right in.

I think when you compare the cost of rebuilding a cast iron Powerglide or a Hydra-Matic (the only autos that could simply bolt up--you have to get the right version of each) to the cost of an adapter and something like a TH350 you might find the adapter is not all that expensive. Maybe you'll find a good one cheap, but its hard to bet on.

I'm concerned about the condition of your motor. If you have trouble pulling past 30 mph you may have more problems than just a transmission will solve. Even with the 5.14 gears 45 or more should be possible with a healthy motor and stock size tires.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...