Yet more heat, on the case, around the plug. Like, with oxy-acetylene
burn tip (minus central oxygen jet of course) until it glows. I'd make
sure the plug was not in contact with the oil, and heat it quickly,
intensely, and carefully.
Especially if it's a cast plug, impact wrench likely will just shear it
Or, drill plug and use extractor. If really bad, you might try
drilling it out to just shy of the threads and then re-tap the threads.
Depending on orientation, you might have problem with chips getting
Some old-timer may have a can of Chrysler manifold heat-control-valve
solvent. Great stuff, with moderate heating, for getting frozen stuff