We did exactly what youre talking about this year, last year, and the year before that. We made motor mounts out of Delrin this year, and aluminum last year. Just hold the front part of the transmission, and the motor itself. When you make mounts, be sure to make it so that the motor cannot rotate no matter what. This year, we made a little bracket that hits against the front of the motor where the clutch belongs. It pushed against a small square raised protion of the transmission, and prevented it from rotating.
If you havent already, lose the clutch, it isnt worth keeping. If you look at the fron of the transmission, with the clutch off, on either side of the shaft is a little metal tab with a spring on it. Make sure that your mount keeps these from falling out, or the motor doesnt run very well, in our experience.
We used some pretty net couplers that work very well. Think of the dogs in 45's, and other transmissions, except with two, longer prongs. These connect together, with a rubber spider (I think thats what its called) in the center. This allows the motor, and/or shaft to not be perfectly aligned, and still run well. We just tapped one of these for whatever size threads onto the output shaft of the drill motor, and then pressed the other one onto the shaft. I think it was secured with pins, but I don't remember.
If you want to save a fairly decent amount of weight,make it so that you are rotating the wheels and not the shaft. This allows you to use only four pillowblocks on the entire drive system. We got away with using bushings too, instead of bearings. You can fabricate your own pillowblocks without bearings, which you can use to mount the wheels in, and keeps the whole thing relatively light. I wont go into the whole #25 vs #35 chain. Choose whatever makes you more comfortable.
We found that for a relatively low speed (3-4 FPS) we could gear the motors down approximately 4.14:1 with a 11t sprocket on the drive shaft, and a 45t sprocket on the wheel, with the drill transmission in low. This gives a lot of torque, but you probably want a bit more speed unless you are going to shift the drill's with a servo, or another way. If you want information on that, feel free to PM me. We have done this for two years, and would like to think that we have it perfected (almost)
Almost forgot. If youre going to not shift, make sure you secure the transmission in either low or high gear. You can find whitepapers here on how to make rings to lock it in place.
If you have any more questions on what we did, or in general, feel free to PM me and I'll try to help ya out.
Cory