I did lots of research
I am starting the tank cleaning here shortly.
The method I will be using is listed below,it seems pretty widely used. I called a few radiator shops and they recommended this also over using creme to seal the tank. copied from another website:
Most recently, I've settled on a Muratic acid wash followed by a dose of phosphoric acid as the best method, mostly because it always gets the job done and doesn't require a lot of physical effort. To do the job you first have to decide if the tank has scaly rust or just surface rust. If the rust hasn't gotten to the point where it's scaly you can skip the 1/4" nuts, referred to later.
Flush out any oily varnish that may be coating the lower regions of the tank with acetone or any of those engine degreasers. If you choose the engine degreaser flush the tank with water.
Remove the fuel level sensor. Seal it's opening with GOOD duct tape.
Remove the petcock and seal it up with an appropriate plug or capped section of hose.
Dump 30 or so 1/4" nuts into the tank.
Pour in a pint or so of Muratic acid. Seal up the fill spout with GOOD duct tape. Wear gloves, old clothes, and safety goggles. Agitate the tank for 10 to fifteen minutes. During this time you can monitor the pressure in the tank by watching the duct tape bulging at the fill hole. If too much pressure develops, carefully vent it by peeling the tape back than reseal and keep shaking.
Next pick a piece of concrete that you don't particularly care about. Remove the tape from the sender opening and fill hole and flush the tank with cold water. The acid that has not already been consumed will neutralize itself on the concrete. You're still wearing goggles and gloves right?
Now dry off the openings and reseal them with GOOD duct tape. This time pour in a pint of naval jelly, or milk stone remover or tile haze remover. These are all phosphoric acid formulations. Phosphoric acid is not as aggressive as Muratic acid and therefore is mostly a waste of time if you're trying to really remove rust. What phosphoric acid will really do well is leave the inside of the tank coated with iron phosphate, which is somewhat rust resistant. For this step you do not need much agitation. Just tumble the tank over several times and let it set for a half hour or so and then tumble it again. After about two hours of this go back to the concrete slab and open the tank and flush it out thoroughly, THOUROUGHLY.
Now bring it back in the shop and secure a blow dryer to the fill hole and force dry the tank. This may take two hours. Position the tank in different orientations during this process to make sure that no seam holds water. Once it's really dry you're done. If you skip the phosphoric acid step the inside of the tank will flash rust before you can get it dried out. I've probably cleaned ten tanks with the method I described and never damaged the exterior paint or eaten through the steel.