If it's frozen up good there's a likely chance you'll damage something even if it does free up. The Marvel Mystery oil is a good trick and may work, let it set overnight. I knew some guys that had a 74 F250 4X4 with a frozen up 390, they actually put it in gear then pulled it with another truck until it broke free. It was a 4-speed manual truck. It ran after that but not very well if I recall.