nice drawings but my opinion would be not to run one bat on the side like that for balance reasons....like i said my opinion only

a 3.3 chassis can easily be modified to run two batt trays and go with a 3905 tranny and your good to go

check mine out...its a 3906 tranny but you get the idea



and this picture shows a nihm on one side and a lipo on the other side just for battery placement mock up