[Three Peaks]

If that is truly a fuel cell, it will have a rubber bladder inside. The problem is the bladder fill will correspond to the fill location on the outer metal tank. I'm not sure you can just reposition the bladder to meet up with a new fill location on the top of the tank.

After going back and looking at the pictures again, I think if it were me I would ditch that tank completely and take some measurements to see what stock OEM tank may fit in that space. Find a tank with a fill tube on the passenger side so you don't have to go through the trunk with the fill tube, or at least not in the middle of the trunk. Then, fab up a tank support, run a 90 degree bent fill tube from the tank fill up through the trunk on the side and up to a fuel filler cap. Then fill in the hole in the trunk and reclaim your trunk space. It would actually be pretty easy and inexpensive if you have any fab skills at all. Even paying somebody else to fab up the tank supports and the filler for the hole in the base of the trunk would be pretty inexpensive, IMHO.

Bob