A lot of guys use either a 66 mustang, or a 66 comet gas tank. The reason being, the way they mount, they use a flange all the way around, so all you need to do is build a frame and set the tank in. The difference between the mustang and the comet is how the filler neck turns, the mustang one goes to the rear, and the comet goes to the side. Here's a pic from the back of my shoebox.
Yeah I got a mustang one in my 53 chevy in the trunk, I was looking for one for a 49 Pontiac to be hung in original spot. Thanks man