That's actually something I'm working on right now. I'm trying to convert a Mazda RX-8, which has a weird gas tank that goes up and over the drive shaft. I think I can get about half my battery back where the gas tank was (which is right under the back seats).

I'm not sure where all the rest are going to go, but probably most will be in the engine compartment somewhere.

I could also use the space where the muffler was, but I'd have to make sure there's enough weight in the front of the car to offset it.