Thanks MetalMan, that'd be really helpful! I've whipped up some rough diagrams of the sort of space there is available on the chassis, though as you can see, there isn't much with either steering servo configuration, and I really don't want to go with 1P. I'm thinking 12 (or 14) cells are going to be pretty hard to cram on there, even when stacked; I can't come up with a decent config. Maybe you guys can! The other thing that concerns me is that, aren't A123 cells kinda difficult to solder? I've not used them before so I don't really know, but I've heard the aluminium can is tricky to solder to?
This is the LiPo layout that I was going to use, and I might do still:
I'd use the RC-M battery tray, with just one big 5 or 6S, 5000mah (or aroundabouts) LiPo strapped in as well as can be. It'd certainly fit better and give better output, but it's just getting it properly secured. I might do a lexan casing around it, *if* I go that route.
Anyhoo, thanks for any ideas.

And SuperBasher, I couldn't agree more lol!