follow the right steps. You could use a gas generator and run a hose from the exhaust into the tank for a few minutes to displace the air. No air, no boom. I once saw a guy do that and use a cutting torch to cut a hole in an underground gas storage tank to install a vapor recovery system. I used the method it fix a 5 gallon metal gas can. I didn't trust myself to do it right so I filled the gas can almost full of water and then topped it off with the exhaust. It worked just fine.