Nice-looking job there, Lake.
If and when I ever get around to finishing my fleetside, I was envisioning a 1976 suburban tank, and grafting a fuel filler door to the outer panel. The suburban had about the same width on the frame rails, I think
It would likely take a bit of work to build in some kind of pocket or cover for the filler neck.
Spectra Premium # GM15A
Gas Tank Capacity : 25 GAL; 95 l
Gas Tank Height : 28 3/4"
Gas Tank Length : 28 1/8"
Gas Tank Width : 10 3/4"
I couldn't read your dimensions in your diagram.
Does this sound close?