Doug, are you using the stock tank, or a fuel cell? If using a cell, what my buddy did with his Mustang, was use a small cell, below the trunk floor sides, and he then made a flat steel plate cover, that completely covered the big hole where the factory tank went thru the floor, and covered the cell as well. He then cut a hole, about 4" diameter, to access the fuel cells filler cap, and then found a large flange with a screw in cap, so to fill the cell, he would open the trunk, unscrew this large cap on the flange, to be able to pop open the fuel cell filler cap. The bottom of a fuel cell has to be covered with metal, to prevent punctures or abrasion in case of an incident, so if you wanted to keep a somewhat stock look from underneath, you could maybe cut the top of an old factory gas tank, place the cell inside , and make either a new cover for the top, or modify the top of the stock tank into a cover, and install an access cap. As for sealing the area behind the seat, and between the inner wheelwells, I wonder if a factory fold down seat trap door would be considered acceptable? Of course that does not address the huge space on either side, between the quarter panels and the outer wheelwells. Thats a large area, between the door jams and the tailight panel, and behind the rear seat interior fiberglass panels, that is wide open.
