The easiest way to achieve this would be to swap in an overdrive transmission, rather than do a rear swap. If you go with the SM319 overdrive, which was a factory option, it is a direct bolt in. This is a common transmission to find, though you may have an easier time of finding one from a '55-'57 Chevrolet car, which isn't a problem as they are essentially the same as the truck version.
I did this swap on mine, and can easily run 75 on the highway.
