I have no idea if you've already read this thread on the H.A.M.B. about a 55 Pontiac gas tank.
https://www.jalopyjournal.com/forum/...ptions.383343/
I'm aware that 55 is not the same tank as a 57, but it has to be close enough that you can possibly make one of these suggestions work for your car. Since there are no replacement tanks for 57, it from the posts it seems that even back to 52, no one makes tanks. Finding something close and adapting it is probably the easiest way out. There is a mention of looking up a tank in the Hollander interchange, says 57 is a one year only, and nothing else made by GM is a direct fitment.
This guy used an older Mustang tank, and cut a hole in the trunk and suspended it the same way ford did. If your car is a nice example, you're probably going to rule out using the mustang tank due to cutting out the trunk floor.
There is also a mention of using a GM truck tank. Maybe this will help, or it may be of no value to you, just trying to find something that may give you some useful info. Good luck.............
