I am trying to think who Frazier fought that punched as hard as Rocky, and Foreman is the only person who comes to mind. I am of the opinion that Frazier was done with his career at that point and physically did not have the same fit presence he had as a dominant heavyweight (see his wide ass vs Bugner, Foreman).
How this fight would play out is difficult, as both men had tremendous heart and power.
If they fought 3 times I imagine it would be split down the middle or the same fight everytime coming to a hard fought UD or maybe a draw. This, for me, is one of the toughest matchups to try analyzing from a fantasy fight perspective.