Thanks guys for the comments.
I kept working at it for a while longer....
Then I had the solid part of the wrench (with no cut-outs, where my top finger is in the picture) wedged against the glow plug and a notch, when suddenly the head gave and turned.
The engine was brand new. The head was exceptionally tight on there. Since I'm using the engine in the Kamdax Mig 15 ducted fan, I'll probably have to remove the whole fan unit and engine just to change a stinkin' glow plug!! [:@] What kind of design is that?? [:'(]
I did buy the adapter from mecoa to put regular long rc glow plugs in there. It fits perfectly! I wonder if the engine will run the same though???? [&:]
http://www.mecoa.com/acc/adaptors/index.htm