I know you said it had no rust, just nasty old gas, but I used a method to convert the rust to an inert material. I filled the tank with a solution of water (15gallons) and a 5 cups of soda ash (ph + for swimming pools) Get a large zinc plated bolt, and your battery charger. Using a large gauge *stranded* wire, strip about 2" of insulation off and wrap the bolt with the exposed stranded wire. Suspend the bolt into the tank through the sending unit hole making sure it does not touch the sides. Attach the bolt's electrode to the positve terminal of the battery charger and connect ground to the tank. Set the charger on 10 Amps and let the system sit like that for a few days. You will see the reaction start to bubble and spooge will collect at the opening. Every day remove the bolt and clean it off, fill the tank with more solution and continue cleaning. It will take several days to convert all the rust to a black inert material.
Rinse tank well.
Chris
