Since gas needs to be a vapour to burn and also needs a fresh supply of air, it can be a little tricky. First thing to do is drain the gas from the tank and refill it with water. Then you are going to need a source of carbon monoxide, so get the car, or truck, running and pipe that exhaust directly into the tank. You can seal it with duct tape but leave the engine running while you are soldering. This will eliminate any chance of fresh oxygen being available to start a fire. This is how it was done in the old days. I actually know a guy who did this with a full tank of gas, since gas as a liquid won't burn. He's still alive and well but I still think it would be much safer with a tank full of water.
Then if there is a small leak, it won't be a combustable vapour.
