Try a bigger hammer!!
Or have you tried to use some liquid wrench or other type of pentrating oil. Also I wouldn't heat it until it is red. At that point the metal grows soft and if it is as stuck on as hard as it sounds it will more then likely bend or deform as you pull. Plus everytime you heat metal you add fatigue if you only have to do this once you won't loose enough structul integrity to be noticed. But something heated and cooled several times will begin to degrade its strengths so be careful and do only it as few times as possible.
