For some reason I can see this going a lot like Louis-Conn 1 whereby Tunney's defense and ring intelligence help him avoid punishment and grab a slight lead on the scorecards going into the championship rounds. However, somewhere in the 13th or 14th, Joe's size and power advantage take their toll and Tunney absorbs a combination that floors him for a 10 count. I know Tunney managed to avoid Dempsey's power but Louis was just bigger, faster and a better combination puncher. And Tunney spent the bulk of his career fighting guys under a 180 pounds. He just wasn't built for 15 rounds with Louis.
I was definitely thinking about Louis-Conn when I posted this MM and I was thinking that just maybe Tunney might be able to close the deal that Conn couldn't against Louis. Tunney wasn't as big as Louis, but he was a naturally bigger fighter than Conn. I also think if Tunney is ahead the fight in the championship rounds, I don't see him taking the chances that Conn did. If Louis is going to rally in the later rounds against Tunney, he is going to have to create his own openings because I don't see Tunney giving him any. I think it would be a very good fight.
I hate to do it but I have to pick Tunney in this one. He was a smart fighter and would know the threat of Louis and as such wouldn't stand and trade with him but would rather stay on his toes like he did against Dempsey...and Conn did against Louis and win the decision.