Tank you very much: advice please

1712290288668.png
Has anyone tried this stuff? Is it applied to the exterior? I'm Googling around for NOS 1960 A Frame replacement tanks, and nothing is turning up. Will a later model Valiant or Dart tank fit? Where do you find these things?

I'm thinking any good light source will illuminate enough of the interior to see if there is any real issue with rust. As far as sediment, the tank can be dropped and cleaned out, I'd think. I've got to replace the sending unit, anyway, and that should provide an adequate window, I'd guess.