Paul:
Just my two cents, but the easiest swap to get an overdrive gear and some better mileage is a 3 speed with overdrive tranny. They were used from about 1955 through about the late sixties. They use an electric solenoid to shift the transmission into overdrive. There were two versions, an early non synchro version with the 4 bolt side cover and the later synchro version with a seven bolt side cover. I have the later synchro version in my truck. Read all about them here:
http://www.classictruckshop.com/club...5n/rebuild.htm
Regards,
Steve
New Orleans