I don't agree with @Max Power wholesale, but gotta admit that Marciano would be destroyed by Klitschko. Not because Marciano wasn't good, but simply that he would be too small. P4P he's great, but with his size, he can't even hit Wlad in the chin. Wlad would lean on him and flatten him by accident in the process. See what happned to Mormeck? Povetkin? It would be like that, but worse. Worse I say!
Even prime Mike Tyson would have troubles with getting held, leaned on, and tied up because of his small stature.

So in summary, Marciano = great tough fighter, but - just - too - small.