With all this effort,,, I'd be simply looking to Rick for a tank. They make exactly what your needing and you can sell the cell you have to offset the price. I mean with everything your buying / modifying / changing here it just seems like it would be better to do let the experts do a tank exactly for what your building. I bought one of their EFI tanks for my 67 Camino resto-mod project. Pretty nice piece.
Here's a link to their truck page.
http://rickstanks.com/truck-gas-tanks.php