Unsticking an unused engine........

You probably don't want to try this, but once upon a time I had a 273 that was junk, and it was frozen up. I decided to get it freed up as an academic exercise. I did all the soaking tricks and was going to take the crank out and get the pistons out one at a time. However, the rotation of the rod bolts was such that one bolt on one piston was in such a location that I couldn't get that nut loose any way. So I turned the motor upside down and whaled on one of the crank weights with an immensely large BFH. It worked and broke loose whatever was seized, but it also mushroomed out the crank weight which I didn't care about.

I'm not sure if this method would bring other collateral damage or not, but it did work. If I was to do it again and wanted to not bugger up the crank, I'd take a hardwood block and use that between the crank and sledge.