Mr Moo,
Is the socket hitting your header flange? take a close look. This happened on my friend Paul Risch's Classic Roadsters Cobra. The wonder boy that built the car for him (destroyed it is more like it) just screwed the spark plugs in by hand because no socket would fit. .....Believe it or not
Steve
P.S.
I had to clearence the header flanges with a die grinder