[cooperino]

The safest way once washed is to use a hose connected to the tailpipe of a running car. The carbon monoxide won't allow any left over vapor to ignite. An friend of mine "guy owned a shop up the street from mine" Blew up his building and killed himself fixing a tank out of a car. Be very careful even if you think you have all the fuel out of a tank.

Edit.. Just thinking of this gives me chills.. Really hits home.

If you have never done this type of repair do yourself a favor. Bring it to a radiator shop. Most radiator shops will fix it for you. They have dunk tanks and can make it safe to weld. Then either let them fix it or take it and try yourself..