I think if these modern fighters went back in time Rocky would wear them all down over those long last rounds. If he came forward in time he would be disqualified for hitting with the side of the glove too often.
Tough mindedness and hard times would weigh against easier times and better food and more technical training. Some fighters are hungry for fame and a name these days but rarely would they have grown up with real hunger and having to work from the ground up. No chainsaws back then, men did 12 hours in the field or on roads then went to train in the ring later. I'll go with the old.