Tanks can be lined...
various suppliers sell epoxy products or kits to coat the inside of gas tanks.
There's Kreem that the bikers use; Eastwood and POR-15 have products; if you look in Hemmings you'll probably find more. Any of these, successfully applied, would address the "thinning" you claim. But I suspect the problem is pit corrosion leading to many incipient pinholes. Anyway, welding up any cracks, followed by the cleaning and coating using these products, will bring you back.
There was also a poster who took apart the two halves of the tank and addressed things in the open, then had it welded back together.