i used meratic acid i got from lowes, did like 30% acid and 70% water. add the water first then the acid, never add water to acid it will basicly explode or splater and may burn you. plug all the lines and set the tank upright so you can get it as full as possible. iused an old sending unit to plug that hole, you will need to find something. it will smoke a little at first but let sit for a couple days and flush out and use baking soda as a nutralizer (flush the tank several times) and get one of POR15's tank sealer kits, works great and seals the tank with a silver looking ceramic like coating, good thing is it will never rust again!
you can take it to a radiator shop and have them boil it out and seal it with urethane (or equivalent) This will also remove most of the old paint from the outside allowing you to attend to the rust with encapsulating paint.
Remove everything from the tank, including the drain plug
Shop will charge around 60-65 bucks. It'll come out really nice.
Cheap trick...get some busted safety glass and pour it into your rinsed and cleaned tank. Tape all the holes up with duct tape, and strap it to the mouth of a small concrete mixer, and turn it on. Change the angle it turns at once in awhile. A few hours later, it's down to clean metal. Coat it with something made for fuel tanks and your done...