Has anyone ever thought of putting a spare on the outside? I thought you could cut out the spare tire "bowl", then welding a large flat plate there, and covering that w/ carpet. Under-mount a full-size spare on the outside, like a trucks spare. Then you would have all the trunk space at your desposal that your car can deliver. This is just a concept I'm throwing out there.
Gas tank is there. But if you end up gutting your interior, you could put a fuel cell where the back seat is then do the spare thing. Something to ponder.