First off, would you mind telling us what brand glow plugs they were? I'm thinking autolites......
Well there are a couple of options.
You could just hammer it into the combustion chamber, and start it up. The odds are against you actually blowing it through without damage, but its been done. I do NOT recommend this method, lots of cussing and hurting of the pocket book ensues...
Probably the most common thing to do is pull the head. Takes a long arse time, but you will get it out. Wont really work if you need the truck in a day or two. If you do do this, its highly recomended up put in ARP headstuds if its a 6.9, with new head gaskets.
You might be able to pull out the injector, rig up some sort of small, small pry bar and put it through the injector hole, and try to push the tip back out. You cant use much effort cause you dont wanna crack the glow plug hole.
Basically if it happened to me I would pull the head. Give me a good excuse to put in headstuds (I really need them cause I am running a turbo). When you go to replace the glow plugs ONLY buy motorcraft or Beru plugs, DO NOT buy any other brand. They are expensive but are the least likely to break off on you next time.
Good luck and dont hesitate to ask any questions!