I believe you are thinking of a 3/4 ton. I have never heard of any successful changouts with the 1/2 ton center sections, but that doesn't mean something couldn't be done, I guess.
Either way, going from a 4:11 to a 3:90 seems like alot of work for an extra 5 mph. When I was in highschool, I changed out the stock 4spd/torque tube rearend in my '53 with a '57 3spd tranny and '58 driveshaft and rearend, and it wasn't really worth the time for me. Using an overdrive will help, but the first gear in most T5's will be pretty useless, if that matters to you. 3:36 gears are available for the 55-59 truck type rearend, but that is your only choice. going with something newer will give you better gear choices.
