On engines that are stuck pretty good, you need to remove one piston at a time. If you have the bearing caps off, can't you turn it in the opposite direction so you are only dealing with one piston at a time? I've never heated an alum. block and pistons, but have done cast iron. If they are junk, try what Tim suggested--break them using obvious precautions. FWIW if you are trying to save them, you need a block turned with a slight clearance on the bore, and with a relieved center. That way when you hit it, all the force goes to the outside (skirt) and doesn't risk breaking the piston. Sometimes time is needed--days or even months to let penetrating oil do its job.
I'm assuming you have taken the valves out of play--I folded up a couple pushrods failing to do so on my crawler.
The grey Case we managed to clear the tractor off the ground trying to jack the last piston out of it.