Ford Motorcraft (Autolite) plugs have the same chrome plating and they will seize in the cylinder head. As I removed the plugs from my 17 Expy with 6k miles, it took some force to break them loose. I can only imagine if I had to remove them at 50k miles.
I only put a dab near the tip of the threads so it would lightly coat all of the threads as I screwed it in. I tighten my plugs by hand and then do a 1/8 turn with a wrench. Never had any problems.
Did you give them a little shot of penetrating oil and let them sit for a bit?
