Start diagnosing the problem by loosening the connecting rod nuts a couple of turns, one rod at a time, and see if one of them might have spun a bearing. Do the same with the main bearings. If loosening the rods and mains doesn't free things up, there's a remote possibility of a seized-up cam bearing, but that's highly unlikely. If loosening the rear main bearing frees up the crank, look into the possibility that the rear main oil seal might have tightened up. An incorrectly installed rope seal can lock up a crankshaft on occasion, particularly if the new Best Gasket seal isn't installed according to their directions.
Don't force anything! it's possible to bend a connecting rod if something has a piston seized up where it can't move. A little patience and a methodical approach to the situation beats brute force any time! Good luck!
Jerry
