The flathead was stuck in a 49 Ford we bought a while back. I removed the heads and soaked the cylinders with PB Blaster for a while. That didn't free them up so I heated the top of each piston with a torch (not cherry red, but just hot), then I sprayed more Blaster and let it sit. Then I took a short length of 2 x 4 lumber, put it down in the bore and struck the top of it with a sledge hammer to shock the piston.

Finally the breaker bar on the crank snout would start to move a tad each way so I kept rocking it and applying more Blaster. After doing that for a while the rings broke free and the engine would turn over with the breaker bar. The process took a few nights of fiddling and I guess it took some time for the Blaster to work it's way down the bore.

I now have the engine sitting and occasionallly spray it entirely with WD 40 and spin it over so it will not refreeze.

Don