[Jared]

I don't think you need to put the hex coupling inside the tube. You could put it in the tube and in the wheel. The sdp si .375 hex sleeves have an id of .625, so you could use it as your drive axle if you want.

If you do something like this, you'll need to make sure that the holes/bearings in the frame as well as the output shaft of the gearbox are concentric, otherwise you'll never get it together.