Run it until fully warmed, drive it a while, then PB blaster or the sort around the plugs, let it cool, ... then pray. Tighten slightly, turn back and forth.

Anti seize on reassembly.

Might even want to warm and PB several times ... give the PB a while to work it's way in.

Good luck.

Think those are bad? Read up on 4.6 and 5.4 Fords ...