Glad to hear a solution was found.

Way Way cool Glad to hear that ya got it going
There are a lot of ways to clean the rust out of an old gas tank. I have use a lot of them muriatic acid seems to work the best for me however it must be used with the most caution.
DO NOT breath, get on skin, get vapors in eyes, put in a spray or use in doors ect. Use proper PPE or common sense. Carefully swish muriatic acid in tank for just a few minutes and then let set for a few minutes. Do not use on aluminum or other non-ferrous metals . Flush well with baking soda and water salutation this will neutralize the acid.
Another less caustic way is to put nuts n bolts or even pee sized gravel in the tank and put in some solvent and tie/mount on the side of a garden tractor tire and drive slowly for five minutes or so.
Good luck.