Flip motor over and fill from the bottom with diesel. Let it set. How about dry ice on bottom of the piston, let it shrink it? Heat the block with the forced air heater (if you got one) then fill the skirt with dry ice pieces. The AL will cool much faster than the #250 iron block. Maybe enough to break it. I would put the crank back in and bolt the rod back up and use a break bar on the crank to pull the piston down when its super cold so as not to crack a frozen brittle piston with an impact or use the air pressure: 100 psi over a 273 crown is over 1100 lbs force. Make sure its bolted to the crank or it'll self destruct when it breaks free and hammers the block on its way down.
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