What's with all the wavy body lines, Pres? Is that thing full of bondo or what?

For the battery, you definitely want something better than 4ga. 2ga minimum, I'd say, with zero (aught) ga being the right way to go. I did a car with supplies from a boat store. Boats run some serious cable, so they usually have it on the shelf. Otherwise, hit up a stereo shop, but it might cost more.

And like Vertigo said, sealed battery if you can swing it. My brother had a wet battery dump over on a sharp turn. The rest of the weekend was spent neutralizing battery acid that had dumped out. Also keep in mind that if you have a hatch, since it's open to the passenger compartment, you'll need a battery box to do any sort of event with tech inspection (depending on how legit the series/rules is/are).

And Paul K's battery tie down is serious business! Definitely consider it.