All good suggestions. Years ago when I was rebuilding a Mopar 360Cu In I installed the push rods and rockers but forgot the timing chain, stupid move after a long day! The engine would only rotate a little until it jammed. I wound up bending a couple push rods before I realized what I did.timing chain breaks valves hit pistons
You could be right. The OP stated he "replaced heads". He didn't elaborate when he discovered the engine wouldn't rotate 360 degrees. He would have had to rotate the engine at least 360 degrees during valve adjustment, so I'm puzzled.D W, I don't think a little adjustment is the cause, the 350 is considered by many to be a non-interference engine, meaning that under extreme conditions the valves and piston should never meet.
I think it is likely that a valve has lost its retainer and has fallen into the cylinder, or, perhaps the OP left a wrench in there.
He hasn't been back, so we may never know.