The extraction tool that I tired was one of those "Chisel Types" is the only way I know to describe them. 4 sided that taper from large to small with a sharp edge on each side for unscrewing.

Just am really worried about going to the next size drill bit up from the 3/8" that I have it drilled out to now, do not want to get into the threads on the head.

Any chance that I could drop the glow plug down into the chamber? That was my next fear while drilling, if I go up another size drill bit, and by pushing on the drill, push the plug down. Any chance of that happening?