Here's a couple of things that come to mind;
Get a couple of 6 volt tractor batteries, and mount them at the front of the bed -- 1 on either side. They can be as large as 2 feet across. Run the cables in series to obtain 12 volts, and remove your regular battery.
Fill the doors with concrete. This works especially well if you're figure eight racing. You'll never need to give a second thought to crossing the intersection -- but others will.
Lastly, you could obtain some pig iron (same stuff railroad tracks are made of). In fact, at least on the old (early to mid sixties) Ford Falcon vans, they put a 2, and a half by three foot by about three inch thick slab above the rear gas tank strapped between the frame rails. Careful removing it while under there, as it weighs about 2 times as much as you'll think it does. This may well be the the best choice, as it'd be a really "tidy" arrangement, and the "slabs" are made to order (already).
HTH
Chris
