Dempsey takes it, his footspeed will be the difference. Jack will be on him from the start and Tunney won't be able to outmaneuver him as he did in their actual fight. Very few people believe that Rocky would beat Louis or Holmes beat Ali had the fought in their respective primes, I'm not sure why being past his prime is not a valid excuse for Dempsey? I'm not sure Dempsey would knock him out though, Tunney was a tough sob with great survival skills and I think he would last the distance, being competitive all the way. Dempsey to pound out a clear decision.
Tunney trained muscle memory possibly like nobody before him. He was exceptionally organised and ready to go back while throwing under nearly any circumstances. I think Tunney should always be favoured over the ten round distance he defeated Dempsey over in real life, under any circumstances. Longer distances, it depends on the balance between Dempsey's offence and defence. Presuming Tunney spends lots of time flat our running Dempsey might get him late.
I think if Tunney fights a version of Dempsey that has his legs underneath him...Tunney will be in trouble.
Tunney has the style to beat him everytime, the movement, angles, speed, jab and counter right completely outbox any version of Dempsey. Let's not forget Tunney laid a beating on Dempsey, he wasn't just sticking and moving in there. Dempsey has very good stamina but he is a bit of a frontrunner.
Tunney was too complete a fighter for the balance to shift dramatically in the other direction, however, if we take the peak Dempsey of 1918-1921 (who had plenty of grease on the wheel), then Gene has got himself a full plate. Jack will not gas, be harder to time and cut off the ring better. In reference to the last point observe the Carpentier bout. The Frenchman was not the fighter Gene was, but one of his talents was scurrying around the ring, turning at the shoulder. Dempsey stuck to him like glue. Setting traps, controlling the pace - most of the factors that led to victory are going to be denied against the peak version. A late stoppage or points decision in which Jack suffers bruising but ultimately drowns his opponents' game plan.
Dempsey in 4. In his prime, too fast of feet and hands and relentless. Hitting power and ferocity unequaled.
I think Tunney had the style to always bring trouble,Gene was way ahead of his time with his style but Dempsey was visually rusty when they fought 2 X and also in the Sharkey fight, his power was there but the movement in his legs had aged because of the inactivity. I would have to believe that a prime Dempsey would be more lethal than the version that fought Gene and thus should have an improved performance, whether that equates to 2 victory's for Jack I am not so sure but the Long count rematch was pretty scary for Gene so I am thinking Jack would bring more and at least finish with a victory.
I suspect that a younger Dempsey would have got him eventually. Tunney might build up a points lead, but The decline in Dempsey's reflexes played a part in his success.
And I will add that all things being equal I don't see any particular reason not to believe many of the old timers who felt Dempsey was one of the best fighters from 1900-50. If I'm not mistaken, Prime Dempsey had more 1st round KO's than pretty much any champion in heavyweight history and while I'm not suggesting Tunney would get KO'd in the 1st round had the fought under the old rules whereby you could just stand there waiting for your opponent to get up, I would certainly suggest a fight under those rules would favour Dempsey and add to the likelihood of him winning by KO somewhere along the line.
Tunney. I don't believe in the myth of a "prime Dempsey". The guy struggled with Brennan and had so damn many soft touches during what should have been his prime that I am led to believe his management was hiding something. A closer bout perhaps but I will still take Gene.