How about the old tow chain in the tank with a few gallons of fuel once you have it out of the car? I always heard that if you remove the float assembly, toss in a big old chain and shake the crap out of the tank it'll clean up the flaky rust. Then dump it all out, flush it good with a solvent, blow some air in to dry it and dump in a quart of good etching primer or rust encapsulator for another shake & dump. Seems like it ought to work, maybe even without the primer?