I would guess that one of the rods is hitting the cam. I think it is either the number 2 or number 5 that must be ground down. If you pull the cam, you might find that it rotates fine. Put a little clay on the top of the rod bolts and you can see where it is hitting. You could try installing the pistons one at a time without the rings and rotate it until you find which rods are making contact. The contact could be at the pan rail. Sometimes they need to be clearanced. Don't get carried away though as you don't want to get into a water jacket .