[Michael C Henry]

If your sure that they are allready galled together? You'll have to sacrifice one of the parts. Preferably the plug. When the hull drain plugs where seized in the alluminum hull of M113's. these plugs were lage about 1 1/4"" NPT with a large 3/4" square drive socket. I'd drill out the center of the plug as far as the wrench socket would allow, then take a die grinder and a rotary file ( carbide bit) preferably a non ferous type so it can be cleaned when fouled. They foul when grindind on alluminum. Then Id start cuttung the remains ot the p[lug intp three or four pieces. The first piece would be a section cut with clearence in two cutts, to be slid inward from the circle creating a gap in the rest of the ring to calasp inward.. Grind untill you see the pattern from the thread shows. I've been known to cheat and then use a hacksaw blade to cut farther into the threads then smack with a hammer and chizle to break that piece loose and remove . Once that portion of the circle of plug is removed one or two more weakened sections can be beat inward on the circle calasping the circle enough to remove the reains of the old sacrificed plug. I'd then go back and retap the housing a little more to clean it all up. Then teflon tape would take care of sealing lubericant, and anti rattle requirements.
Next time use a steel or brass plug. They are stronger than the alluminum and if they get stuck you can smack them to free up or heated to expand then wait till cooled down .. When you heat the plug it trys to expand pushing the outer housing, especially alluminum, then shrinking as it cools, giving freedom the plug didn't have before. One you start transfering metal as in galling it is no longer a simple unscrew it any longer. If you have seized , forcably twisting the plug will just ruin both plug and housing.