This is just my opinion but I personally would get rid of the tranny alltoghther. Its gonna give you nothing but problems. I would look into a buggy center diff or an outrunner direct drive setup. The direct drive setup would be perfect for a 12s lithium setup since thats what their meant to run at. If you want proof that that tranny isnt strong enough well then this might change your mind-
http://youtube.com/watch?v=J_VWhdE4HRk
And to think that this is only 6s. 12s is gonna be one helluva strain on the tranny.