Largely depends on how you rate the ability Sullivan and the best of his era, compared to how you rate the ability of Rocky and the best of his era. Personally, I think that everything we know about Sullivan tells me he would probably be a very tough matchup for Rocky to overcome. Bigger, maybe even much bigger in some matches depending on how you interpret the newspapers that say he looked fit at as high as 220. When he went 76 rounds with Kilrain he was estimated to be around 207-214 lbs so he still had formidable cardio at a heavier weight. Also probably much faster, Corbett said that the prime Sullivan of 1884 was as fast as Dempsey in his 1925 autobiography, and the version of Sullivan he saw fight George Robinson was estimated to be between 204-219 pounds. That means Sullivan was likely much faster than Rocky despite being potentially 30 pounds heavier at times. For the same reason I think Dempsey is a bad matchup for Rocky due to being an early starter, and having so much power and speed, I think the same likely applies here. However, there are many unknowns regarding Sullivan’s durability against someone as powerful as Marciano, as well as how he would hold up against some with such a high workrate if he doesn’t stop Rocky early. Everything we know about Sullivan does suggest he also had world class toughness and heart though, just read about the conditions of his rematch with Mitchell and even his heart in the loss against Corbett, I think he had as much of a will to win as Rocky himself.
