Maybe use a USB endoscope to view inside the tank ($15 ebay). If just surface rust, you could pour in phosphoric acid to slosh around and sit for a few days. Wash out with water. Add bolts and stuff to shake around and break up the loose converted rust (iron phosphate). Works well for me on rusted car parts. For lots of nuts and bolts, I shake them in a jug w/ water and keep repeating many times until I no longer get black particles. Cheapest is probably "concrete etch" at Home Depot, but in many other products like Evap-O-Rust. If you were near-by, I have a spare fuel tank (rust-free CA).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans