I assume the cylinder head is off? If so, pour 2-3 ounces of KROIL into each cylinder, and let it soak for a week or more. Put a catch pan under the crankcase, as the oil will work its way through the rust and drip out the bottom, and it can be recycled and poured through again. A little heat with a Propane torch to the tops of the pistons before you use the penetrating oil will help the oil do its job.
www.kanolabs.comYes, it's expensive, but it works very well to free up rusted piston rings. After a few days, use the chisel end of a long shepherd's crook wrecking bar on the flywheel teeth where they intersect the bottom of the bellhousing, and rock the flywheel back and forth a little at a time. If it gets really hard to turn, back up and move the other direction. Eventually, you'll be able to make a full turn on the flywheel. Be patient!
Last resort- - - -if the crank simply refuses to turn, take the inspection cover off the top center of the bellhousing, and use some universal joint sockets, extensions, and lots of foul language to unbolt the pressure plate. Then you can remove the flywheel and bellhousing.
Jerry