[dsraven]

how about a custom (or not) round fuel tank like a lot of highway trucks use. it could be a as simple as buying an air tank that fits the space and then welding in the appropriate fittings for the filler and outlets and some mounting hardware (or use straps to hold the tank into some saddles on the frame).
check under a newer mustang for driveshaft clearance, not much there either. I know because I helped a buddy install some new exhaust on an '87 era. skinned knuckles to prove it. the important thing is to know your rear axle cannot move over enough to hit anything. a good panhard bar could be in order and then run the suspension through a full up and down to ensure it has clearance.