whats the best way to get old varnish out of a tank?

Muratic acid (you can get it a pool supply store in the chemical section) and some rocks or a 4'-5' length of chain. Make sure its sealed up so nothing drips out and then spend the next day or so shaking the crap out of it. Be sure and do it in an EXTREMELY WELL VENTILATED AREA!!!! After that hose it out REALLY WELL. Get the inside dried out immediately!!! Do a final cleaning with rubbing alchohol and be ready to do the tank sealer of your choice before any rust or corrosion begin to form and ruin all of the work youve done to get it ready. Most A-body repacement tanks rund about 140-150 plus shipping so you may want to weigh the cost of just getting a new one versus all the time and expense of cleaning your old one. Tank sealing systems arent cheap. Have fun and good luck!