First thing to do is to pull the fuel guage sender unit from the top of the tank. You can then look into the tank and get an idea of the condition.

Worst case of course it a totally corroded interior which could mean a tank replacement. You can get the tank 'boiled out' I understand. Perhaps radiator shops offer this service.

If the tank can be cleaned out reasonably well, but still has surface defects (coating off in places) - then coating is an option.

My origional tank was trash when I pulled it. The tank I got from my parts car looked really good. I just gave it a good pressure wash, and when dry - gave it a treatment with dilute phosphoric acid just in case. I'd recommend the acid treatment before coating if that's the route you go.

Good Luck!