[keg65]

To clean the tank, you'll need a pressure washer, some MEK to dissolve the varnished fuel, and maybe some muratic acid to clean up any rust. Wash out the inside of the tank with the pressure washer, most of the gummed up fuel should break lose, but some of it will be like paint and really stuck. Methyl Ethyl Keytone (MEK) is highly flammable and fumes are bad, but it does work. Let it soak on the areas with the varnished gas. If the tank is rusty inside, some muratic acid will take that away without eating away the metal. Rinse out, blow dry with compressed air. I used to use a torch to dry, but that can be risky.
replace all rubber fuel lines and fuel filter. Odds are the carb will need to be rebuilt too, since the varnished fuel is in there too.

Check your rubber brake lines, and it if has been sitting, then pull off the brake drums to check it the wheel cylinders are leaking. Brake fluid attracts water, which settles to the bottom in the cylinders. Most likely replacing them is best.

Also do what you need to stop the rust. Don't just cover up the floors with new sheet metal. the rust must be stopped.

Everything is quite simple to do, but very necessary to have a safe driver that will last and be reliable.

Good luck and congrats