FWIW, I installed the under-seat tank from a Jeep CJ-5 under my 57. It was NOS, so I could cut and weld without fear of a big bang. I had to plug the original filler hole in the side, and fabricate a new filler on top in anticipation of a gas cap in the rear fender. I modified the mounting holes and pick-up tube from the original 57 sending unit to fit the Jeep tank. I got a pair of new hanger straps based solely on proper length. The T-tabs attach to the crossmember over the rear end. I installed a new crossmember of 1 x 2 square tubing about 8 inches from the end of the frame rails, and that carries the threaded ends of the hanger straps. The new tank rides tight up against the original rear cross member with pieces of tire sidewall for tank-to-frame buffer. I does indeed look like a suitcase hanging there, and way too much work, but infinitely cheaper than a new 69-72 'Burb tank. PS-if my geometry calculations are correct, it holds about 23 gal.
