Tunney UD. Tunney had no issue going 15. I think he could go 25 rounds. Unlike Johnson Tunney was more mobile, and had an even better jab. Who has the better wind in rounds 30-45 is anyone's guess, but I think the action would be light.
Intriguing matchup. Tunney definately would have been forced to fight in a manner that he was unaccustomed to in order to pull it out. This one could be problematic for him. Johnson was a defensive stylist but he could switch it up and turn aggressive which is what he did against Willard, leaping in like a bulldog to throw volleys of punches and stay well ahead on points. Ive never really seen that Tunney could do that, especially against a much larger guy like Willard. Another thing is that Johnson was a very strong guy even for his size and this strength served him well against Willard. I dont really get the impression that Tunney a very physically strong guy from his fights. It definately would have given us a good handle on another facet of Tunney's game.
The Willard who fought Johnson was far better than the out of shape Willard who lost to Dempsey. Still, Gene by decision.