Stylistically, this is a nightmare match up for Rocky.
Marciano was not as fast, explosive, didn't hit as hard, and wasn't half the athlete Tyson was. Tyson actually weighed between 215-220 at his peak compared to Marciano's 185.
The disparity in weight and natural ability would be too much for Rocky to overcome. Not to mention, Marciano came right at you, and didn't know how to back up. That is suicide against someone like Tyson. Rocky just wasn't big or strong enough physically to fight Tyson that way and be successful. Also, if you look at the way Marciano dodged punches, he often put his head in an area where Tyson could get the most leverage in his punches.
Tyson almost decapitates Marciano with an right hook to the body/right uppercut to the head combination to end it in the 3rd round.
Source(s):
Official for USA Boxing