With a 4.82 rear end ratio, you can't go very fast in an M5 truck. Overdrive is the way to go! Find a T86-1D or T86-1E transmission from an M or R series truck with a Champ 6 engine. You can't use one from a truck with Big 6 (Commander engine) or a V-8 because the input shaft diameter and length are different. The overdrive ratio is 0.7:1, so you can cruise at 2400-2500 rpm at 60 mph with overdrive, depending on your tire size. Overdrive gives you five useable gears: 1st, 2nd, 2nd overdrive, 3rd, 3rd overdrive. It's better than changing the rear end ratio to 3.31-3.91 and probably cheaper. It is very difficult to find the ring, pinion, and the ring gear carrier for transmission ratios of 4.27 or less.
The column shift mechanisms are difficult to find and most of the old ones are worn out anyway. Most truck guys want a floor shift. So, take the floor shifter mechanism from your old 3-speed box or get one from an early Jeep with T-86 transmission. The older T86-1D transmissions had the same location for the 6 holes on top as the 3-speed transmission, but the later T86-1E may have the middle holes displaced. If so, drill a new set of holes in the floor shifter cover to match the transmission.
Disassemble your overdrive transmission and take out the front shifter fork (2nd-3rd) from the side. Put a plug in the hole that seals it well. Take out the rear shifter tang (reverse-1st) and grind off the top 1/3, cut the shaft shorter (because you won't need the outside shifter arm) and replace it in the case. This piece is what will lock out the overdrive when you shift into reverse. After checking and cleaning gears and bearings, reassemble the transmission. You may actually have to do this a couple of times to get all of the clearances right on the shifter forks and tang.
Take the rear shifter fork from the floor shifter and grind off the left side until it looks like this:
You may also need to grind down the thickness of the front (2nd-3rd) fork so that it doesn't keep forward pressure on the sliding gear and synchronizer when you are in 3rd gear. It's difficult to see how much clearance you have. We used a flexible borescope inserted into the transmission through the filler plug to see how it looked with the cover on.
One issue I encountered on my T86-1D transmission was that the opening in the top of the case is narrower than the old T90 3-speed case. That means that even if the bolt holes line up, the top cover doesn't seal against the normal gasket. The top of the case is 4.5" wide on the outside. The gasket sealing area on the T90 cover with the floor shift mechanism is 4.5" wide along much of the inside of the cover, except around the bolt holes. The front and back edges are OK for making a seal. When I put the standard gasket on, oil came out the sides of the joint. I made a new gasket out of thick material about 1/4" wider on each side so that the cover would be able to pinch the gasket at the edge of the case. I hope it works well enough so that it doesn't leak too much oil! I filled the transmission with Pennzoil "Synchromesh" gear lube, available at most auto parts stores. It is designed for transmissions with brass parts and won't eat them up. It's equivalent to 40 weight engine oil and close to 90 weight gear lube. The closest gear lube to the 90 weight mineral oil lube originally recommended by Studebaker is type GL1, but it is hard to find. Some farm stores and truck parts places may have it. You can also use non-detergent 40 weight engine oil, but this is also difficult to find. Do NOT use type GL3 or GL5 gear lubes because it will eat up the yellow metal parts!
The top of the T86-1D case showing the width of the machined surface.
The floor shifter/cover from the old T90 transmission. The areas inside the 4.5 width are recessed and won't clamp the gasket
The M5 trucks with overdrive from the factory came with an extra bolt-in crossmember. It supported the weight of the overdrive unit hanging out the back and may have prevented the tail of the transmission from kicking around under hard conditions. Later trucks with the same transmission did not use a crossmember, so I don't know if its absolutely necessary. In any event, the M5 frame has pre-drilled holes for this option.
Unfortunately, I took the picture above with the crossmember installed left-to-right. The cut-out is supposed to be over the exhaust pipe!
Here are the drawings for the crossmember and cradle:
Cradle drawing PDF
Crossmember drawing PDF
Since the overdrive transmission is longer than the 3-speed, the driveshaft has to be shorter. You can either find a original driveshaft or get a new one made. The length of my driveshaft is 59-5/8" between the centers of the U-joints. This is for the 1946 and older T86-1D transmissions. Note that later model T86-1E transmissions are slightly different and may use a driveshaft length of 58-13/16". The older transmissions use a slip-joint yoke, while the newer ones have the U-joint flange attached to the transmission with a large nut.
Then you need to wire up the solenoid to the overdrive relay, which gets mounted on the firewall. M5 trucks came with an extra wire in the main harness to connect to this relay. The relay is to the left of the voltage regulator in the picture below. The resistor you see is for the Pertronix Ignitor connection when you have overdrive.
Factory-installed overdrive also had a kickdown switch which was contacted by the throttle linkage when you floor the gas pedal. I couldn't find the special linkage parts, so I wired in a switch under the dash to the right of the steering column. I used some 1 x1 angle iron to make a bracket for the switch. [Soaking hot-rolled angle iron in white vinegar for a couple of hours will remove the scale without grinding it off.] I just flick the switch when I want to go out of overdrive, like when climbing a steep hill. You have to remember to flick it back when you get to the top, though. You also need to install the pull cable to lock out the overdrive completely. It goes to the far left of the steering column. Original cables had a large ivory-colored knob. You'll also see in this picture, at the far left, a starter button I added. There is also a starter relay on the inner fender liner that is connected between the battery and starter. Pushing the button pulls the solenoid in on the starter relay. If you ever had a Studebaker stall and fail to restart from vapor lock, etc., this kind of starter connection lets you put the truck in 1st gear and use the starter to move the truck off the road.
Studebaker didn't provide a rubber boot to seal the shifter opening from the wind and water. I think a boot from a Jeep CJ2 or CJ3 might work, but I haven't tried it yet. Many of the details of changing over transmissions was covered in Turning Wheels in March 2006. This is not a 1-day job, but it is well worth the effort of the change. Happy motoring!