The reason why I ask:
1. Obviously the spare tire will be covered
2. 16 Gallons sloshing back and forth, behind your rear axle.
= surprise pendulum drifto
(think winter too... will make things worse)
3. 100LBS extra in the trunk = motorboating = not so much 50/50 weigth distribution = more understeer.
4. Rear end collision = fireball. (see Ford Pinto) Big
5. Higher center of gravity = turn like poop.
6. Filler neck on the same side as the (hot) exhaust + drip while filling up = gas on the exhaust pipe = fire hazard.
Maybe I'm a little anal about this stuff, but thats what I was getting at, and to me, they're all valid points. If this car isn't already in your name (ie, safetied), you'll be in rough shape convincing a mechanic that your car is safe/legal. If you're doing all this engine work, at least reconsider putting a new oem/aftermarket gas tank where it ought to be. Even between the rear shock towers, I think you're making a mistake. Otherwise, like I said, nice job!