Been there, Done that too !!!
NUT is right... Thats what Ive had to do in the past...
Give up on the pistons, as they are gonners anyways...Use a good quality penetrating oil on the cylinder walls and flip the block upside down... You need to do this on the ground and on plywood, as you are taking a risk doing this proceedure on an engine stand...
REMOVE THE CRANK FIRST!!!! Believe it or not, you can seperate all of the connecting rods from it, take the main bearing caps off and remove the crankshaft. You need the crank and dont want to hurt it more than it may already maybe...
2) Get a BIG BRASS BAR similar to a punch, but not pointed on one end... Get yourself a BFH (Big Friggin' Hammer) Place one end of that bar onto the piston itself directly below the pin give a couple of good smacks, (IE driving the piston UP towards TDC) move the BRASS BAR to the opposite side of the piston...give the hammer a few more smacks... If all else fails, get a bigger hammer...
What this does is rocks the piston back and forth as you DRIVE it out of it's bore... You end up beating the hell out of the piston but not its pin, and no damage will be done to the connecting rod...
Your block will need a rebore anyways so dont worry too much about what the rings are doing to the walls. Just be sure to use a BRASS BAR , because if you slip you dont want to strike the cylinder wall with a hard metal like steel...
Rockfish