3.07:1 rear as found in the 57-59 gmc 1/2 ton 3 speeds will be a bolt in deal, however any 50 year old rear end will likely need a refresh.
Now for the numbers... your rear end currently should have 3.90:1 ratio. You can go to any online gear calculator and use this information + tire diameter to figure out the rpm at any given speed. A 235 can spin up to 3200 rpm all day long, however 22- 2400 rpm should be about ideal at 65 mph. The calculators will solve for desired gears if you put the rpm and the speed in and leve the gears blank.
