Gas tanks are one of the hardest items to find for these old panels. Even if you have a good clean tank the pickup tube inside it really needs to be redone and moved towards the front. We like to have them sit low in the front and that means you need to keep enough gas in the tank to not starve the pickup tube. In a stock tank the pickup tube goes straight down from the opening in the side. Been thinking about maybe a mustang tank in the rear as you get more space back there than you do in a pickup and could probably still hang a spare tire. At least that is one of my thoughts. I've heard a custom made tank is in the $800-1000 range. I sent a panel truck tank out to a place in Pennsylvania I think and they open it up and blast it then weld it back together and cover with some black goop. Came back looking like a black football barely fit in the brackets it was so warped and bulging. I'm sure it won't leak but it was not pretty. Worst money I ever spent.
I did think about a flat square tank under the back, but fill and spare mount shut that down. A 53 has the shape that would fit inside frame but the fill would take work, it's on the top could move it to the side. Most of the universal tanks are small. I'll come up with something. I appreciate any and all input thanks. MW