Dave,
I have the Trick/Flow twisted aluminum heads on mine and a regular spark plug wrench won't begin to work easily. So I just went down to the local hardware store and bought a deep socket that size and it is about half the thickness and it works great. Also as Steve mentioned, check that you aren't hitting a header flange.
CORRECTION -- After reading Rick's post I went and looked and I got my thin wall plug wrench at NAPA. The deep socket that i got at the hardware store was to get to some bolts on the Comet.
Ron
