Marciano wears him down and stops him late in a brutal war of attrition. Both fighters are never the same after.
**** I picked the wrong option. Dempsey by KO is what I meant to choose. Dempsey is so underated it's crazy, people swear he was a crude slugger when the reality is he was one of the innovators of the bob and weave and was very good at slipping punches and picking off shots as he attacked relentlessly for as long as it took to wear his man down and take them out. Dempsey was the most vicious fighter in history with excellent subtle skills, Marciano also had good subtle skills that were overlooked but Dempsey was better than Rock in just about every aspect and had blazing speed, he'd get to rock first and Rock was hittable and able to be knocked down by far lessser punchers at HW. Not to mention Dempsey, although smaller than most Heavyweight all time greats, actually has a decent size and an enormous 10 inch reach advantage over Marciano.
Dempsey has more skills and hits just as hard. It could very well be the greatest HW dream match in history. Check out the recently found footage of Dempsey in training for his fight with Carpentier in the Classic section, it is unreal.
man what a fight this would be! I have a hard choose choosing who would win, so ill just go with a draw. but man if only we had a time machine.