My old 216 was locked up tight and I tried everything that was mentioned above with no luck. When I finally pulled the head one piston was the culprit (number 3) the others looked like new. Since it had a cracked block I salvaged what parts I could but could not get it free until I banged it hard with a hammer and a wood block. Moral of the story you never know what you may find in a 60 year old engine/truck.