If you try and run the engine to warm it up, you do run the decent chance of removing the threads.
It sounds like you may have the original 20some year old spark plugs in the engine. I've seen it before.
If the Craftsman stripped head socket tools do not work, you could try and tack weld something to the remaining part of the spark plug. Sometime the heat of the welding will be enough to break off the frozen-ness of the threads. Pulling the head will solve your problem, because you can bust out most of it and torch/mangle/drill out the rest. At this point, you are pretty stuck, so you could try and start it up and get some heat into the head this way. You are probably going to take out the threads anyhow, a helicoil isn't too hard to install, and if you are planning an engine swap anyhow....
Also, say you do fuck up that head, S12 heads aren't that hard to come by. You could use that one to practice porting on, and then port one you get to replace the old one and bolt it in!