Joe Fraziers heart could fight in any era and he used his size and style to his advantage most of the time but I believe that Marciano's two handed attack would be the difference. Rock had power in both hands and was a more compact puncher utilizing more punches than Joes arsena had. Joe would need to be at no more than 212lbs. the older 220lbs Frazier would get hit alot more than the younger lighter Frazier. People who think Marciano wasn't deliberate and calculating are wrong! He knew how to judge distance as he felt out his opponents and knew if he could get under or slip outside on them. The so-caller brawling style is not without techniques and Joes lack of a jab as he got older and heavier would make things easier for Rocky.
If Ali didn't have his size (length) with his jab Marciano would KO him, as Ali said after their computer fight He'd never felt punches like Rocky's before. Now they weren't fighting but what Ali was saying was simular to how "hand shakes" are different from man to man. You can tell when a man has "natural" strength from his hands and any fighter can tell by light shadowing or light sparring with someone just how strong their touch is and that gives an idea of power! I like Marciano in a fairly close decs. in 10 & 12 but TKO win in a 15 late in 13/14/15!
Ali in great condition weighing no more than 215lbs goes 10 &12&15 with a close win but if he's hurt at anytime he gets stopped! Marciano has the conditioning to finish a guy who's hurt and will not be pursuaded to back off. Just look at his nose split down the middle wide open and how he kept coming!! Again the men like Marciano & Dempsey seem much smaller than the heavies that came after but I stood next to Ali & Marciano & Ali & Louis and their seemed to be a little height difference. Granted Louis & Marciano & Dempsey were long retired and above their best fighting weight but there just wasn't that much difference. At that time I met them Ali was about 220 and coming off a bout of the flu, so he wasn't exactly peak but his shoulders weren't any wider neither was his neck and his hands were smaller than Joes & Rocky's.
Dempsey I met by himself and he was about 225 I'd guess and he looked BIG wrists, neck, shoulders, head, hell even his face looked big hahahaha now that way over his prime weight but this guy held his later life weight perfectly so imagine him at 185 to 192 same as Rocky and Louis at 200lbs!!!
Throw in their intensity and willingness to win and their individual talents and you have great competitors who fought at a high level. The Highest! Ray.