I agree that removing the parts and welding them on the bench is the way to go. Rendering a tank safe to weld near, is a very difficult and time-consuming process. In warm weather an "empty" tank emits petrol vapour, so the region around it sometimes contains an explosive mixture. If it ignites, the small explosion can envelope the tank, causing rapid temperature rise and the emission of a lot more petrol vapour, which sustains the fire. Then air enters the tank, an explosive mixture develops inside the tank, and you get the famous earth-shaking kaboom.
