I think the TP 1320 is about as big as you can go in the O.E. location. I'm using a DN Power 1350. It's about 69mm at it's longest point (including wires) and it does slightly interfere with mounting the body.

Also if you do mount a battery in or below the frame try and get it as centered as possible under the mainshaft and it will minimize cg issues. I personally wouldn't do it because of the added weight and the added labor of switching out two batteries after every run. I am curious to see the length of the run times though.