I've had good luck with our local radiator shop. If you can find one in your area, they will dip it in a strong solution and it will come out real clean. Keep in mind that the paint will also be gone.
After that you can line it with something like Red-Kote sealer and you'll end up with a rust free gas tank that is essentially a plastic tank.
Do follow the directions to a 'tee' and it will work.
(I have nothing to do with this company except to have enjoyed using a product that does what it says it does, for a change.)