You could fit 12 cells per side pretty easy.
I am running 8 Li ion cells per side at 6000 mah.
4 li ion cells I have are almost the exact size as a 6 cell pack they just are little smaller around in dia..
sub C cells are ROUGHLY 1" in dia and the li ion cells I have are ROUGHLY 5/8".

I dont know why people still run sub c's when you can get li ion for about the same cost (unless you aleady have them).

Big Daddy;
The spur gear came from my LHS. I am not sure where to get one online. If you go that route I have a few tips...
Get some shims for your center diff and make sure you have a good fit before you put it all back together. The plastic spur is real close to the metal one but I had clearance issues inside my center diff and had to remove the shims that were already installed. Also pick up 4 of the pan head type screws for the diff spur you will need them. The screws that mount the spur on need to be FLUSH on the spur or you will have clearance issues when you mount you motor plate.
Hope that helps