If it's an A Series. (no overhead cam), go buy a head gasket and remove the head. Don't mess about with anything else. It'll take an hour, and save an engine rebuild, and/or replacement cylinder head. It the thread into the head is buggered, they'll need to put a helicoil in, which is best done with the head off. Also with the the head off you have more tricks to try in regards to extracting the remainder of the sparkplug.