Marciano was fortunate he fought when he did, everyone knows that. His record will always say he was an American heavyweight professional boxer who was undefeated 49-0.
He fought from 1947 to 1955, just under nine years. He didn't have any big physical advantages, he was just very determined and could punch with either hand.
Was he the greatest heavyweight ever? No! Would I like to see how he would do against today's heavyweights? No!
That record will always be 49-0 (43 KOs).