I have a broader question. What was the cause of the corrosion on your fuel tank, and why was it so bad as to cause a leak? This makes me wonder what kind of preventive measures should we be instituting to prevent this from happening on our own boats?
Since you have the tank out, measuring it will be easy. ?Surely there any other manufacturers of marine fuel tanks who could fabricate this? ?I don't know if $500 is a reasonable number for something like this, but I'd go with a newly fabricated tank given the trouble of getting it in and out of the boat. You'd be good for another 25 years with a new tank.