[vectorit]

Just because you didn't mention it...

Are the heads cast iron, or aluminum?

Either way, another reason why I coat the threads of sparklers with anti seize.

If they are aluminum, you are in trouble.

If they are cast iron, then brute force will probably do the job. But then I have seen cast iron threads get galled from seized bolts and plugs being forced out, so I would run the engine to get it warm and try to break the seize. Then spray penetrating oil on them to help loosen up the rust.
Let it sit and cool, and turn tight and then loosen. Run the engine to warm up, then loosen again. While they are on the outward turn, spray more penetrating oil and tighten then loosen. Repeat until the plug comes out, without taking thread material from the head.

Lots of experience with this problem, and this has always worked for me.