I am still considering putting in the 3 speed and switching to floor shift, but if possible I would like to keep the four and add overdrive. That way I would get the best of both worlds and get to keep my 7.05:1 first gear for towing and have the 0.85:1 overdrive to get my rpm's down and my mileage up, plus I would just prefer to have 4 gears available. The other problem I have is that the 3 speed is from a 1/2 ton and the 4 is 3/4 ton, so they have different sized yokes. I've tossed around a couple different ideas about this before, converting the yoke on the 3 speed, swapping the rear end and driveshaft from my c10 parts truck, or modifying the rear end to accommodate the smaller driveshaft. After doing a little more research I found info on adding the overdrive to a four speed, but as Longbox55 said, looks like it's only a simple conversion when you use a 4 speed from a car. But, it seems like it would still be possible to do with my 420, it would just take a little more work. The next step is to take apart my transmissions and try to figure out if/how I can make it work. Theoretically I think I could make an adapter to mount the overdrive unit behind my 420, or have one of my buddies in the machining program make me one. After that I just have to figure out what driveshaft/rear end to use, and shorten the shaft. For this application I would be comfortable shortening it myself, since it won't be behind a real powerful engine.
