This is a really good thread...A couple of thoughts.
First, Tunney was a very underrated puncher, and, possibly, one of the smartest and most disciplined fighters in history. I've read where the quality of Liston's jab was established by saying it was "nearly as good as Tunney's." In the accounts of his fights you read often of his tremendous body-punching skills, and he would NEVER have done what Conn did in the first Louis fight. If Schmeling was clever enough to hit Louis all night with that right, Tunney would figure him out,too. But not in that same way because Louis had that figured by the end of the firsat Schmeling fight, it was just too late for him. Schmeling hit harder than Tunney, at least, I think, with his right, but he was not nearly as thoughtful in the ring. And, not to speak bad of Louis, but the rap against him was always that he was robotic in the ring. He had trouble adjusting.
Marciano was pretty damn good. Goldman always said that, if Marciano had started boxing at 12 instead of at 25, he'd have been the greatest ever. He said his biggest problem was in not trying to teach him too much. His power and his stamina and determination should be unquestioned, and, if you really watch his fights, he's a bit more clever and harder to hit than given credit for. This is where I think he would do well against most modern heavyweights: he advanced with his weight back on his right foot, prodding with his left. Being short and in a crouch, his weight being back made him a bit harder to hit then a guy like Frazier who came forward leaning over his left6 leg while looking for the hook. Marciano's stance would make a guy reach for him.
Think, for a second, about a modern day giant of a heavy trying to jab the 5' 10" Marciano while he he crouched back like that. He can't reach him, unless he reaches for him, because there isn't one of these guys smart enough to get low and jab like Foster did against Tiger. When the tall guy reaches and jabs down, what punch does he get hit with? That winging right that was Marciano's best punch. I submit that his lack of height and his style would present problems to today's super heavies that would only be matched by their inability to deal with his work ethic.