I had a nice original gas tank in my 1971 Barracuda, except for all of the varnish, gunk, ethanol residue, etc. throughout the tank. I gave up on cleaning and replaced it with a new Spectra tank. I saved the original in hope of one day being able to clean it....completely.

I would not put anything abrasive in the tank. The original tanks were made of terne plate steel. There is a lead-tin coating on the steel that was use to prevent corrosion. Don't damage the coating on your tank.

I would also avoid taking it to a radiator repair shop to have it hot-tanked. And stay away from muriatic acid. Both of these will damage the lead-tin coating.

David