I haven't done it yet but I'm going to put an over drive unit in my truck, the redneck way, but it should look and work good. I bought a 4speed tranny out of a 1950's 2 ton international truck. IT has very similar gear ratios to my 47 chevy. What I decided to do involves putting the tranny in backwards behind the original. I'm going to take off my middle mount and install it between the two drive shaft pieces that I'll have to have shortened.
I'm going to linkage the shift from it up to the cab through the starter button hole (my engine was up graded to a newer electrically activated solenoid) and have it so I can shift it between 3rd and 4th gear.
4th gear will give me my standard speed and 3rd gear will give me a .59 over drive, cruising 55mph at close to 1900rpm.
