Spinks was good enough to last the distance with Dempsey ala Tommy Gibbons. I see him breaking up Jack's rhythm with his sneaky punches and unorthodox footwork. Dempsey by decision(tentatively).
If we're to take the Spinks who fought at/near LH, at his best, I think he wins this fight. He did a wonderful job of nullifying the attack of a smooth, skilled pressure fighter in Dwight Qawi- and did so in a situation where his common-law wife had just died. So, that version of the Jinx isn't going to be so easily daunted by the prospect of having to face someone like Dempsey, who while talented and hard-hitting as all hell could still be troubled by movers like Tunney and Gibbons, and still be shaken to the core by the accurate and quite powerful punches that Spinks tended to throw. Spinks whethers some tough moments, but keeps Dempsey off-balance, scores a knockdown, and scores a close decision, imo.
Bigger, faster, sharper, and more skilled than Dempsey. No comparison to be made in my judgement. I think Dempsey gets outclassed on his way to getting knocked out late. I've seen bits of footage of Dempsey, and I appreciate the fact he was successful during his era, but the sport has evolved since then. I think Spinks has the size, strength and skills to keep Dempsey off of him, or to at least minimize the damage done. Like in true Spinks fashion, once the opening 4 rounds are over, he usually steps it up a notch and throws in combination and works in spurts. Dempsey has no answer for the sharper faster punches of Spinks, who would get there with his right hand late on in the fight. I think Dempsey probably wins the early rounds, Spinks is one of the slowest starters ever, but the difference in quality will tell.
Dempsey gets under him and hammers his body for a quick stoppage win. Jack dropped a rock hard Bill Brennan for the count late with bodyshots. Granted, Spinks had no business being in the ring with Tyson, but still, it might be revealing that he was dropped for the first time with a right to the heart. I just don't see Michael surviving long with the likes of a peak Dempsey or Frazier. When the solidly constructed Dempsey undertook training for Gibbons, he weighed 236 pounds (the exact same weight Louis topped out at while in the military). The spindly Spinks scaled only 170 pounds for Sears, his penultimate LHW contest. (For comparison, Fitz weighed 167 for Corbett.) Check out Frazier-Foster. How would Dempsey-Spinks turn out differently? At 37, Gibbons was badly faded and compromised when Tunney finally retired him, yet stood up to a tremendous beating until worn down enough to be floored for the first time in his career in round 12. When Dempsey decisioned Tommy, Gene did not believe he was yet ready to take Gibbons on. To survive against Dempsey, Tommy did some smothering in close. (Like Dempsey, the tough Gibbons had some quick knockout wins over modern sized winning heavyweights.) The taller Spinks wouldn't likely be smothering Jack like that. Michael wasn't able to stop an out of shape and undertrained Mustafa Muhammad or Eddie Davis. (Davis was stopped five times in six defeats.) Yet some believe he would manage to take out a peaking Dempsey? Starting slowly with Dempsey would do him no favors here. Like Jack, the lightning fast Carpentier was a quick starter, but that didn't help him. When Tunney dethroned him, Gene immediately stunned Jack with a devastating right hand, and broadcaster Graham McNamee went on to announce to the radio audience that Tunney was, "out Dempseying Dempsey." No version of Michael was ever going to start out doing this to a primed Dempsey.
And this happened, what? Once in his career against a peaking monster? Care to describe Jess Willard? Was Dempsey superman? Well if you insist, but that's not one of my better qualities..