Philly, We are talking about Ali in 1971, not early in his career pre-title. Louis is too slow-footed and hittable. He struggled vs. fighters who can box and move ( as champion ) ,none of whom were in Ali's class. I'm sure this fight would break all PPV Records, and I'm pretty sure Ali would win on points, comfortably. While Frazier did beat Ali in 1971, Frazier had dynamic feet with the ability to bob and weave his way in to close the gap. Louis doesn't have this ability.
WRONG! Louis could fight a multitude of styles. If you want to watch Louis bob and weave look at his bout with Lee Ramage. Louis had 10x the skill of Frazier and he would be fighting an Ali not in shape to fight an ATG at his best for 15 rounds.
The problem as I see it is I don't think Louis could put the pressure on Ali that Frazier did. There'd be a lot of exchanges at range and that favors Ali.
Joe Louis, Ali lacks the footwork reflexes, speed, and stamina when he came back from his enforced inactivity, due to his draft evasion conviction on June 20 1967. FOTC Ali did not have any of those attributes that I have mentioned, or else he would given Louis a boxing lesson, like Billy Conn did in 1941, before he made that fatal mistake of slugging it out with Joe, and getting kayoed in round 13. Louis beats FOTC Ali by unanimous decision like Smokin Joe did the night of March 8 1971.