No different than the 3 speed that is in there, really. The final drive willstill be 1:1. You would also have to change the bellhousing because the 48-55 years used a different bolt pattern for the transmissions.

Your best bet is to lose the torque tube and swap out to an open driveline. It opens up many more possibilities. You don't need to get an overdrive transmission if you find a rear end with high gears like a 3.07 ratio. Many peolpe on the board have swapped T-5 transmissions from an s-10 in there prewar pickup. I believe that is one of the best choices for getting your old truck up to freeway speeds. The t-5 works best with lower gears... 3.55-4.11. Some people run higher gears, some lower. A stock 235 may have trouble pulling a hill with higher gears.

Cheers
John