Ali weighed about the same as Barkley did against Duran who the later beat when he was a shot 38 y/o, so in theory the size disadvantage shouldn't be an insurmountable obstacle for a prime Duran to overcome. So now it just comes down to a battle of skill and will and fact is Ali never fought anyone remotely close to Duran's skill level whereas Duran actually beat a more skilled opponent than Ali in SRL. Sure Foreman, Frazier, and Liston were all formidable punchers, but they were big slow plodders and Duran was more skilled than all three of them combined. One of the most important factors to take into consideration in this one is Ali wouldn't have his obligatory speed advantage against a prime Duran and speed was his bread and butter and if you take that away from him he becomes a lot easier to beat. I doubt it would be easy because Ali was no walk in the park for anyone but I think a prime Duran could pull this one off on his best day.