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 in two.
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 into lube.
Some old-timer may have a can of Chrysler manifold heat-control-valve solvent. Great stuff, with moderate heating, for getting frozen stuff "thawed."
HTH, J