Marciano would have at least as much of a chance against Ali as Frazier did, probably more so, in my mind, because he was harder to hit. I know the common thought is that Marciano was a punching bag and Frazier some type of defensive genius, but that is simply not the case. Angelo Dundee, Lou Duva, Joe Walcott, Ezzard Charles, Archie Moore and Roland LaStarza all commented on how much harder it was to hit Marciano, in actuality, than it seemed to be just watching him. Most specifically, he would be hard to hit with jabs and right hands because of the way he moved forward, with his weight back and to his own right. Frazier, conversley, was right there to be hit and thus had to bob and weave to avoid punches that Marciano would simply have ignored, being out of range. The place where Marciano would be most vulnerable to getting hit would be inside, and he'd take that exchange and win it all day. There is no way on earth that Ali can be considered the same quality of in-fighter as Moore or Charles and they could not win that battle with Marciano.
Ali would find himself having to reach with jabs and to land his right hand while constantly having to move into Marciano's right hand to do so. He would simply surround Ali's best punch and make him pay to get in a position to land it. Charley Goldman would see to that. Marciano would win a wide decision, going away, or maybe get a 14th or 15th round tko.