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

I got a 64 Valiant Signet for my wife, and the gas tank has 8 year old gassy varnish in it. I will pull the tank, and was wondering what is a way to get the residue off.....My thought is maybe a gallon of laquer thinner, and a bag of glass marbles.....roll it, shake it...anyone have a process they used? Help!!! And, thanks!!
Andrew/Kidd