If it's really stuck, forcing one piston down with them all connected to the crank will try to force others up (and down.) A less destructive way is to pull the crankshaft out first and work on each piston individually. The crank can be removed without getting the cam and front cover off the engine. There's enough room in the front plate to get the crank gear out once the bearing caps have been removed. I would have done that on the frozen 235 I got except the clutch, bellhousing and flywheel were still attached. I pulled the crank gear, freed up the cam and removed it and the front cover. I still destroyed one of the pistons that was REALLY rusted in place. You could also just disconnect the rod bearing caps and drive them out the top with a long punch against the piston pin boss, although that's probably going to destroy the piston as well.
It's kind of like an interlocking puzzle, but the more parts you get out, the easier getting the pistons out will be.
