Try and find a local radiator repair shop that deals with antique gas tank repairs, and has a suitable tank to 'boil out' your messed up tank.
My tank had so much varnish in it from sitting 15 years, you could smell it from 100 feet away and the local
trick' painter said to clean it before he would paint it for $700.
Brought it to 'Duracor' radiator shop in Prospect, Ct., where it was submerged in their tank for 7 days, then sealed with a proprietary red goo.
The original paint and graphic scallops and pinstripes were un-touched (lacquer paint).
Best $100 ever spent, going on now 4 years.
Some things are better left to the pro's - if you can find them!
Edit: Don't forget to put a new gas cap on after sealing the tank.
The old ones develop a ton of fine rust dust up inside with age.
In my case, cheap insurance looking at $400 + with new Mikuni carbs.