I welded a hydraulic tank a few months back, I drained out as much oil as we could, then power washed what was left. Before welding, I ran a hose from the tail pipe into the tank opening. This removed the oxygen, so no oil explosion!
With todays modern gasoline, a few days laying out in the direct sun will dry out all the fuel and vapors. Ace Hardware and I'am sure other stores, sell fire extinguishers in small spray paint sizes cans. These are nontoxic, Halon "type" which when sprayed removes all oxygen from the area. They would work great for this type of welding.
joe
