One time I put a clutch in a truck, and it wouldn't turn through after words. It would only turn so far. It was a straight six also. Finally had to pull the head and found that carbon buildup had been so bad that it literally fell off the head onto the top of the piston. This engine had sat for a good while before I installed the clutch. Might tyr some air into the spark plug holes and see if you blow anything out. Otherwise, I'm betting you will have to pull the head to find the problem.
