If there are no rubber seals in the tank, try a pint or so of that nasty carb dip (the really awful stuff) in the filler neck. Swish it around for a few minutes then pour it off. The stuff is great at getting any petro-chemical goo off (and will destroy any rubber seals) but will not touch rust. A short extension on the fuel outlet and a little dip poured in it and allowed to sit should eat through the goo in the tube in short order.
Once the goo is gone rinse several times with mineral spirits (paint thinner... the real stuff, read the label).
If you want the tank to last forever, get yerself a tank slushing kit. The cleaners in it will get rid of all rust and the coating will resist any attempt to create more by nasty moisture in the tank.
Or, you could periodically add gas dryer to the tank to run moisture through the engine.

Mike!