I usually weld a nut on in these situations. For that one I'd use like a nut for a 3/4" bolt. Plenty of meat to weld on and plenty of surface area to grab with a wrench. When welding, set your ground clamp to the nut to lessen the chance of arcing from the plug to the threads and tacking them together. This could really mess up your threads when you back the plug out.
