because that will relieve any top end stress and avoid any valve to piston contact (where applicable )if it is out of time. plus you wont bend anything if it is. i watched a guy spin around a 440 with a cheater bar that was out of time, and right before our eyes we watched the pushrods bend.
with all of that out of the way, there is no reason this engine should not spin freely. of course unless theres a bad bearing. the only thing outside of the bottom end rotating by itself without any resistance is going to be the timing chain to the cam, and the oil pump / distributor drive. just for kicks, put it neutral