With all the spark plugs out and transmission in neutral, you should be able to turn the engine using the fan blade. What year engine do you have? Those rockers look like the ones used on the SBC V-8's that get adjusted at the rocker studs. I don't know when GM went to that style rockers on the 6 cylinders. Likely after 1958 as I had a 235 with rocker shafts.
