if possible warm the head, get it good and hot, fairly close to operating temp, then chill the plug shell, (a metal funnel with the spout passing through the plug and touching all sides, pour cold water through the funnel, cools only the plug) should loosen it up enough to turn out.
If I have to split the shell I prefer making three evenly spaced splits, less likly to mar the threads while working the plug shell out, and it's easier to get a smaller section out.
If available, drill and left hand tap the shell, then thread a left hand shoulder bolt into it, when the shoulder hits it will start turning the plug out.
or since the head is off tap it right hand and use a shoulder bolt from the combustion chamber side to turn it out.
Hopefully the plug was cracked or weak, and it will come right out once you are able to get some torque on it.
easy outs can be a lifesaver, but by design they 'spread' the bolt you're attempting to remove, and may jamb it in tighter.
not using anti-sieze, or installing cold plugs into a warm engine can lead to this.
never torque a cold plug into a hot engine, (or the reverse) just snug them in. when they cool the aluminum shrinks up more than the steel and the plugs locked in.
If you need to do a 'hot' change, just run them down finger tight, and let them sit a couple minutes to equalize before you snug them in.
I always use antisieze, fresh application about every third plug remove/reinstall, don't need a lot just enough to form a sheild.
Ken.
