Define "extremely difficult", please. Are you using the flywheel to turn the engine, or some type of pry bar, etc.? It's not uncommon for new rings to make a crankshaft noticeably harder to turn as the drag of more cylinders accumulate. Have you checked the clearance of the rods as you assembled things? I'd suggest backing all the rod bolts off a turn or so after installing the final piston, and checking the oil clearance on each rod as you torque them up. There should be little or no difference in the effort required to turn the crankshaft as the rods are torqued. The ideal oil clearance should be .0015"-.002", with .003" as an absolute maximum. Good luck!
Jerry
