I have used the 3 step Eastwood tank clean and coat system with good results. Strong detergent, acetone, acid and sealer. After using the acid I poured it in a plastic bucket with limestone gravel. Left it there for a day or two then dumped it out.

Agree after acid cleaning, a coating would be important.

From a cost (time and money) a new tank wouldn't be much more expensive but the old tanks and sending units seem to have less problem with sealing and guage accuracy so saving yours probably makes sense.