Depending on how badly it's rusted, things like valve lifters and/or piston rings can make an engine difficult to turn. I'd soak the cylinders and lifters down with penetrating oil like Kroil, PB Blaster, or an ATF/Acetone mixture for several days, then either spin the engine with the starter or use a pry bar on it. With the head off, you can also roll the pistons to the bottom of their travel and use a hone in the upper part of the cylinders, but then you have to do a thorough cleaning job to remove the residual hone grit. Once it makes a couple of full turns, it should gradually free up and turn more easily.
Jerry
