If you have any PB blaster that stuff works great, if not, i would loosen it were its safe, stuff some WD-40 down there, slowly work it back & forth untill it feels free enough, don't rush it though. On the new plugs, i would put a few drops of oil on them before installing.

75 Duster, 451 10.87 @ 123.58 NA 97 Z28 6sp., 12.01 @ 115 on a 100 shot 71 Swinger. 360 magnum. 12.58 @ 105 78 cutlass, 469 BBC. 12.70 @ 108 on street tires. http://www.youtube.com/watch?v=Z2kqNmMfheU