Piston rings rusted to the cylinder walls, and/or valves seized in their guides. Pour a few ounces of Diesel fuel into all of the cylinders, and loosen the valve adjusting screws a couple of turns each. If the flywheel is still bolted to the crankshaft, use the chisel-shaped end of a long shepherd's crook wrecking bar between the starter gear teeth and the edge of the bellhousing. You can get a lot more leverage on the crankshaft that way, and you'll probably break the rusted rings loose. While you're at it, figure out a way to pry the rocker arms down against the valve stems a little. That will determine whether or not you've got seized valves. I'm betting on piston rings.

No matter where an engine stops turning, there will be a few valves open. This allows air (and humidity) to cycle in and out of those cylinders as the temperature changes. Next time you store an engine, put a few ounces of motor oil into all the cylinders, and then plug off the intake and exhaust manifolds or the ports in the head with duct tape to keep air from getting to the cylinders.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!