Hi again,

Is the plug completely frozen in place?
I'd forget liquid nitrogen.

There's elements of judgement and risk with problems like this.
Did you try the overnight MMO soak and it still wont budge?

You could try a 3/8 Breaker bar and give it some medium sharp taps with a medium
ball peen hammer. If it moves a little, tap it back. See if you can
get some easy motion back and forth. put it where its free the most and give a
few drops of MMO, then go at it again. What's wrong is the threads are Galled,
and you have just what I had. You'll need to tap the plug holes afterwards.

Just hope you don't snap off the plugs in the head. That's ugly, but so is where you are now, since there was never any anti seize grease applied.

Have at it, Bill