Welcome to the world of old metal tanks. What I did for mine was take the tank off. Go out to the driveway and get some rocks (seriously) put a few pounds of them in your tank + a little water and shake the hell out of it for a while. There is various rust dissolvers/converters and the market for that too.
www.eastwoodco.com sells them specifically for rusty metal gas tanks. One more trick to do is run a tube though your petcot. That will stop rust from being drawn from the bottom of your tank. Definitely use a fuel filter...PB