The glow plug closest to the radiator on the right side in my 82 K2500 wont come out either--it unscrews,but the tip must be badly swollen,I've even gone as far as chucking the glow plug in a drill,and spinning it in reverse,till I feared it would snap off inside the cylinder..I screwed it back in and said friggit..
It seems to start ok on 7 working ones the past 12 years..
I've read how some have simply unscrewed the glow plug that refuses to come out,and just start the engine,and eventually expansion and the force of the compression will blow it right out of the head...so far I haven't been brave enough to try that method...they do sell "glow plug extractors" too,but I doubt they would pull it out without breaking the tip off..
I've had an Autolite 1110 glow plug fail in the #3 cylinder and get chowed in the piston & valves,it hasn't seemed to have done any permanent damage,but I do hear a tapping lifter type sound ever since,until the engine gets fully warmed up...
I no longer "mix" different brands or part numbers ,all the glow plugs need to "match" from what I've been told,otherwise the one with the least resistance gets most of the current,and can overheat and fail--I think it's true once one fails,the rest seem to not last very long too..the 9G's and Champion brand ones swell up the worst when used with manual activation..
I like the AC 60G's but I've had 2 sets of them rust away and look like salted brake bleeders in just a couple years...my exhaust manifolds are crumbling away too,the road salt gets blasted right on them by the front tires...it was great fun removing the rear most ones with bolt out extractors..I was lucky they came out easy,and I used anti-seize on them..