I think it would be a great fight. Joe would probably build up a lead through the first 5-6 rounds, but in the middle rounds Rocky would start to close the gap more and more with his punishing blows.
Louis would continue to consistently land crisp, hard punches throughout the fight, but their effects on Rocky would be limited. Rocky would get swollen, but Joe would be taking plenty of hard hits himself and would start to get worn down by the later rounds. In the championship rounds I think Rocky's constant pressure would finally get to Joe and result in a knockout, with Louis ahead on the cards.
Louis himself said he didn't like to be crowded, and Marciano would be doing nothing but crowding while throwing his famously powerful punches. After their fight, Louis said it hurt to just bump into Rocky (which I believe). Even though Louis was far from his best days when he fought Rocky, he was hardly a push over. Louis still had his power and strength, he was just lacking the reflexes that he once had. Marciano, despite being the smaller man, was pushing Joe around during the fight with no problems. You can see in the fight (especially the 8th round) Marciano was nudging Louis backwards with ease.
Had Louis won, he'd have gotten a shot at the title. He was still collecting good wins when he fought Rocky. Marciano didn't want to take the fight because Louis was his idol, but he saw that he had to do it, so he did it. He cried after the fight because he was sad he had to beat up his idol the way he did.
Anyways, my official prediction would be a late KO win for Rocky, or a close decision win for Louis.
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