Steve,
There is no easy way to do this with the tank in place and do it right. If it turns out you can't get the old tank fixed, go to the top menu and mouse over "Market" then click on "Equipment Suppliers". You'll see two tank sources there with appropriate prints. Bob offers sound advice in his post. We used to weld on full fuel tanks on the submarine with all sorts of special procedures.... you don't want to go there and need to completely void the tank of all fuel and traces of fumes.