I had to break the pistons out of a 440 that sounds similar to your situation. It takes awhile.......big hammers and chisels. Not fun!!!

Start on the worst looking ones. After you get a couple out you may be able to turn the crank!

70 Road Runner 383-4 4 speed FJ5 & black guts
70 Charger R/T 440-4 4 speed FJ5 & white guts