Brian’s tips with the boiling water and to only use a soldering iron and not a blowtorch anywhere near the tank is the way to go. I worked as a boilermaker on ship refuelling barges, road petrol tankers as well as vehicle and boat fuel tanks at times. I also used oxyacetylene and arc welding inside petrol farms which can be rather stressful. We used steam for several hours on road tankers and then checked for fumes. After all the washing out and the sniff test, put an air line passing a small amount of air inside the fuel tank while you are working on it. This will remove any fumes which can reappear from any trapped fuel inside seams etc. as no fumes one day does not mean there will be no fumes the next day.