Here a FYI for anyone thinking about using the 3905 shaft in the G1 transmission. It needed some mods done to the gear and the shaft is 37mm longer then the revo shaft. It does sit the spur right next to the motor mounts. Here's a link to a post with some details the G1 3905 mod is at the end of the thread.
http://monster.traxxas.com/showthread.php?t=412386