This is one over best questions ever posted on sailnet. I going to start by telling you I don't have a good answer. Aluminum tanks are known to leak at the welded seams as over the years water gets in to the weld and causes corrosion and the weld fails. It happens more with aluminum water tanks, But can happen with fuel tanks as water is next to imposable to keep out of the fuel and sits just below the bottom of the fuel pick tube.
Hard call with a 35 year old tank.