Rebuilt motor won't turn

PURPLECUDA340 said
Lower end of my 1971 340 rebuilt by shop, I had to assemble upper end. Assembled cam, , cam sprockets,crank sprocket, timing chain (dot to dot) and was able to turn engine over with the crank bolt attached to harmonic balancer. Once I attached the new hydraulic rocker arm assy, I was unable to turn motor over manually with the crank bolt, even with 135 ft lbs of torque.......If I loosened the rocker arm shaft retaining bolts, I was able ,again ,to turn engine over with the crank bolt.........

Anyone have a clue as to what might be happening ?

Sounds like your timing chain is out of sync and an open valve may be hitting the piston. Sometime back this happened to me. Everything looked right, but the wrong timing chain was in the box I bought. I have a Desoto hemi and somehow another chain was in the box. The Desoto uses a 318 timing assembly. The mark on the big gear was way out. Now I double check everything from autozone!