I assume you have managed to remove other pistons & rods. Now loosen rod bolt nuts on #8 & turn the crank slightly so rod journal ( crankpin ) is trying to pull rod/piston down bore. If you have managed to remove crank already you can skip that.
Clean any carbon of the piston crown. Now grab one of the old unwanted pistons minus rings & rod, set it upside down on top of #8 with the pin in line with the crank c/l . Hold a piece of scrap steel against the piston & give the scrap steel a good solid clout or two with a reasonable size hammer. Piston should start to move down bore, Turn crank as before and repeat process until piston is approx 1.500" down. Then scrape / sand bore to allow piston to be pushed out top.
It is a waste of time and likely to split /damage bore to try and remove piston out top of bore without doing this first as rust forms on piston ring area which has a smaller dia when cold and the skirt area will jam when you try to push it thru if not removed.
I got quite good at this as I worked at an Import Shop who brought lots of V8 motors into NZ from USA in the 70's some of which had been left out in the weather with no carb etc.
Good Luck,
Jac Mac