Some of the guys listed weren't even power forwards. Espo was big and strong but not a power forward. His skill was more important then his strength and toughness. Real power forwards are guys whose #1 attribute is their strength/toughness and willingness to use it. The good ones have some skill to go with it. I never saw Beliveau play but I don't think he was your typical power forward. I don't think Clarke was a power forward either.
I agree that he was more of a power forward then some of the guys in this poll but IMO he wasn't a prototypical power forward. The Rockets game was based just as much on skill and speed as it was physical play. Even if Rocket hadn't been very strong he still would've been a star most likely.