I can't help you with the size of the fittings on your mast (C&C 34).
I can say that I've seen wire rope wear sheaves down pretty badly in our out haul which is internal to our boom. For this project I replaced the sheave (on the boom) with a rope designed Delrin sheave from McMaster Carr: McMaster-Carr
and I replaced the wire rope with a fairly high tech, low stretch line.
Might be time to convert to an all line halyard and replace the sheave.
Check out the picture.