Here's what I'm thinking:
Build a frame up. Raise the floor to make up for the extra height of the frame, reinforce the uni along where the frame will run, and just drop the car onto the frame, similar to a truck. I've been looking at just pics of general car and truck frames, and they don't look THAT complex...obviously alot of work would be involved with building a custom frame, but it doesn't seem like that big of a thing.