Kim;
Awesome truck. I love those fleetside beds.
I have a 58 Suburban that has the original 3.90 gears in it. When I bought the truck it had the original 3 speed trans. The tires were a tad on the small side, so I ran about 3000rpms at 55mph. When I got it up to 65-70 it was almost undrivable. The engine held up fine it was just "screaming" going down the road.
I installed a 3-speed overdrive transmission from a 67 step side pick up that I tracked down on Ebay. The later model overdrives have 7 bolts on the side plate and more importantly come with a synchro first gear. The earlier overdrives have 4 bolts on the side cover and don't have the synchro first gear. I went with slightly larger tires and can now run the same 55mph at around 2100rpms. It will run 65-70 comfortably without winding out too much. If you want to go the overdrive route, shoot me a privat message and I'll share some lessons learned with you.
First thing you need to do is determine what rear end you have. If you have a tach in the truck, see what RPM you run at 55mph. There are calculators that let you put in the tire size, rpm, speed etc. to determine the rear end ratio. If you have a 3.90 or 4.10 an overdrive is the way to go. If you have 3.38, an overdrive isn't the way to go. You won't have enough torque to make the overdrive useable accept on really flat roads with no headwind.
