Here is the best layout I can come up with with 3 separate 2s packs (Dummy packs are 10mm longer than actual packs). I think this would keep the left to right balance fairly close. The dummy motor appears to fit perfectly. I'm having a friend machine up the pinion carrier which as you can see will have to be extra long. The electronics (receiver,speed control and brake servo) will be mounted on the top plate.
I would really like to mount the motor using some sort of "band clamp" type mount. So far I haven't found anything any link/suggestions?