Interestingly both of these two fighters actually weighed the same for nearly all of their fights and what makes it better is that they are both great undefeated champions, personally if I could make one fight through the ages this would probably be it due to their really exiting styles and the fact that an 0 would have to go.
Not even a match Marciano would destroy Calzaghe. There is no way that Calz would be able to stop Marciano in his tracks...he would keep coming and hit Joe with hellacious shots. It would be over by the 8th or 9th.
Joe gets knocked out. Joe likes to stand in front of people and throw thousands of ineffective sloppy smacks to win on the score cards. He stands in front of a slugger with the determination and punching power (not to mention stamina) of Marciano, he goes down.
Calzaghe UD or stoppage (on cuts), by boxing of the back foot and basically making contact with Marciano's face with his glove many, many times. To beat Calzaghe you have to step back and counter. Anyone who steps forwards to Calzaghe is going have his face contacted by a glove many times. This is a horrific stylistic matchup, Calzaghe is not the kind of fighter who can be simply broken down.
Rocky would ignore any and all punches and brutally ko J.C. in 6 rounds. This bout would make no sense for either man.
This fight makes a lot more sense than some of the other ridiculous ones posted. But considering the disparate weight classes they fought in, it would be a hard fight to pick because of Marciano's stamina and Joe's boxing skill, and also legendary stamina, if you ask me (Joe is one of the few fighters that could probably go 15 or 20 rounds like the old timers). Joe could probably keep his distance though and outbox him. You made a mistake in this fantasy setup. You forgot to make clear if they were fighting in a 12 or 15 round fight.