Tunney has a pretty big reach advantage. But then again Rocky always smothered his opponents. So i think Rocky wins by stoppage round 8-9
Dempsey was no longer the "Mannassa Mauler"when he fought Tunney,he had been out of the ring for three years,and living the soft life,prime Dempsey beatsTunney imo.
Tunney was a safety-first, thinking fighter, student of his opponents, who approached each rival as a puzzle to be solved. Marciano found Charles, Walcott and Moore in front of him more often than not because a) they were boxer-punchers b) their old legs could carry them away only for so long. Tunney was a pure boxer whose main goal was the W on his record. He was athletic enough, savvy enough and fundamentally-sound enough to avoid Rocky's best enough to win the 15-round decision.
I don't know about this one. This might actually be a bad matchup style wise for Marciano. Tunney was fast on his feet. Only Ali was a better dancer at HW and Gene was pretty tough to boot. He survived a series of brutal bouts with Harry Greb and learned from them as well. The Dempsey that fought Tunney was still faster than Marciano and Gene took all but one round in both fights! Gene cut Dempsey's eyes badly in the 1st fight and we all know how easily Rocky cut. I think the speed and mobility of Tunney would be the deciding factor for one fight anyway. In a 3 fight series Marciano would probably take 2 of them but Gene could certainly take 1 of them either by lopsided decision or stoppage on cuts.