Have you thought about installing a Moeller Marine fuel tank? I am planning on replacing my old tank with one of these. Since the original is untreated aluminum, and for what ever reason that Wellcraft decided to foam the tank in causing the tank to corrode even faster.
Here are the links:
http://www.moellermarine.com/oem/fuel_tanks/fuel-tanks/
Do you have the measurements on yours? I have a 1990 Fish 18 and I was told it has a 50 gallon tank.