Hi,
I would just get a new tank. They are pretty inexpensive and then you wont ever have to worry about it again. Rusty tanks are a pain in the rear, it seams they never come clean and you will always be cloging up fuel filters. trust me its not worth the hassle.
I hope this helps,
Nick
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69 Z/28 X77 D80 83000 original miles,DZ 302, M-21, BU 373 Rear End, ZL-2 Ducted Hood, Daytona Yellow, Black Deluxe, Tilt, Factory AM/FM, Console Gauges.