I'd have to go with Chuvalo on this one. Chuvalo was bigger than Marciano and had a better defense than most fighters give him credit for. The fact that he was never knocked down makes people to mistakenly believe that his defense was bad, but no fighter with a bad defense can go 97 fights without being knocked down. If Foreman and Frazier couldn't do it, Marciano sure couldn't. I think George throws much better combinations, especially in his later years. Witness him vs. Cleveland Williams: http://www.dailymotion.com/video/xid...williams_sport
I think Marciano would just get overwhelmed. You have to remember that Marciano didn't have very much great opposition. The major players that he beat, viz. Joe Louis, Ezzard Charles, Jersey Joe Walcott, were all quite old at the time. An unbeaten record can't hide the facts.