We have run into the no space for fuel tank problem and sometimes you have to think outside the box. On some we have used several small tanks instead of one large. Sometimes, but not often, they can be mounted at the same level and will gravity feed to the one with the fuel pick up. Usually it takes a transfer pump if you don't want the filler tube to be in an ugly location. We put a small "catch" tank where the factory filler can be used that has the transfer pump connected to it. The outlet from the pump can have a flexible hose that is routed to wherever the main tank is located. The main tank has the fuel gauge sender in it. The regular transfer pumps are slow but if you use a 240gph after market pump that's 4 gpm so it only takes 4 minutes to fill a 16 gallon tank. Is there not room at the rear of the frame to mount a tank? You could move the filler from just in front of the fender to just behind it. I have pictures of a 52 Suburban we did which has a Lincoln IRS that the torque arm on the right side took up gas tank space. Would send pics if you thought they might help.

Evan