In modern boxing it is a distinct possability that Rocky would not have been as succesful.
It is also a distinct possability that his work ethic and dedication could have made him great, but perhaps at Light Heavy, given modern training, conditioning and nutrition.

Horses for courses.
In th era of the smaller heavyweight. Where stamina, power and resilience were the key rather than size, power and chin, Marciano was King.
He would be so succesful now, but then again one couldn't rule out wins against Chagaev, Imbragimov or the second Tier, could they?