Converting Duster gas fill to behind the licence plate?

One thing I loved about my 1970 Cuda and 1971-1972 Chargers is that I could pull up to any gas pump and fill my mopars with not having to deal with waiting for the right side pumps to be unoccupied. Since my 73’ Duster is needing a trunk pan from the spare tire being rotted out, i was thinking this might be a good opportunity to change that. Decided to convert the rear to being completely flat so I could mount a fuel cell. I was thinking since it would be flat, I could mount it under the car, giving more room for battery mount and storage. I could cannibalize a few cars for parts to do it, but wondering if any members here have done the conversion or might have some nice suggestions.