If the crankshaft rotated easily by hand before installing any rod and piston there is really no need to loosen the main caps during your trouble shooting, remove one of the last two pistons and rods and see if that free the crank up, if not remove the other one and see if you can find the issue
It is ill fitting or a twisted ring probably
Good luck Lets us know what you find, that will help others on here now and in the future