The best thing to do is to remove the tank if you can, failing this, fumes are your worst enemy, can you drain the tank? this may sound strange, but if you can drain the tank and refill it with water it will be safer, third option is to cover the tank with a couple of layers of wet fabric, not nylon, and a final cover of a canvas tarp and have someone standing by with a fire extinguisher. Happy welding.
p.s. if you take the drain and fill with water option, flush the tank with methylated sprits before you put fuel back in, this will absorb any residule moisture.