A clash of two great heavyweights, Iron Mike Tyson and Rocky Marciano, would be like most pay-per-view events—vastly overrated.
First we must give credit to Rocky Marciano, the former center on the Brockton High School football team who became one of the great athletes of his era and retired undefeated, with a 49-0 record.
Rocky Marciano, unfortunately, lived in an era when he did not have to face any fighters who even came close to matching his level of ability. Consider that his wins over Joe Louis, Jersey Joe Walcott, Archie Moore, and Ezzrad Charles were all against fighters (often smaller men) who had all seen better days.
Marciano broke you down over a number of rounds. He wouldn't know how to deal with a fast starter like Iron Mike (this was also a problem for Joe Louis). Marciano's technique was often sloppy. He simply outlasted his opponents.
I could see him doing the same against Tyson, but he wouldn't have gotten the chance.
It's more likely Tyson would have used his speed, power, size, and even reach (Tyson actually has reach on Marciano!) to force Marciano to fight hard from round one.
Marciano's punches were often sloppy (re-watch the Cockell fight for proof), and Tyson was a great counter puncher. In addition, Marciano only faced one 200-plus pounder in his entire career: a faded Joe Louis.
Tyson is so much bigger than Marciano and quicker than anyone Marciano ever faced. If Archie Moore and Jersey Walcott can make Marciano kiss the canvas, you could bet that prime Tyson as well as prime Jack Dempsey could have done the same.
I could also see prime Muhammad Ali and prime Joe Louis winning on cuts versus Rocky Marciano.