Many have had this trouble. Filling thru the inside of a vehicle is not my idea of a good solution. Filling an aftermarket rear tank from outside is OK (rear wheel well or new body hole). I have thought many times about all this. Someone could stamp these tanks in identical halves (almost) and join them. The tank is made like the truck tank. I have often wondered if a 55-59 truck tank ($140), could physically fit under there. If so, modifying the filler and sensor hole would be cake. Modifying a new tank, that will fit in stock location, would be easy and totally safe.
Also the various tanks for Fords, 32-40 approx look interesting.
I have also looked a big truck (18 wheeler type) round aluminum tanks for mounting in original location. I say all this to maybe spark an idea in someone, it's called brainstorming. For the aftermarket rear tank, spare can be stood upright and mounted to the side or mounted flat with a nice cover on it.
