I would be getting the soldering iron hot, tipping the tank so the contents is away from the joint and resoldering it. Dont use a flame near the tank, with a flame heated iron do that well away from the tank but a electric iron will be fine. You will need a large body iron or high wattage electric.
You may end up completely removing the pipe, thoroughly cleaning and refitting to get it to seal fully.
1938 1/2 ton Hope to drive it before I retire