So I restored a fuel tank for my 70 Polara 9 passenger wagon.
I had the rust chemically stripped...
Before...
After chemical strip...
Unfortunately this process revealed some holes/thin spots...
My father and I brazed them up and coated them with solder. I then used POR15 fuel tank sealer for the inside...
After that I applied POR15 to the outside of the tank to finish it off.
The chemical strip was $300
Already had POR15 black on hand as well as brazing rods and solder.
The tank sealer was like $35 on ebay.
This is about the best that can be done unless a clean used replacement can be found.
Good luck with your tank, I hope this helps.