Larry Holmes did actually name Jack Johnson, as the historic fighter who would have given him most trouble. Not hard to see why. They both fight exactly the same fight, but come at it from completely different directions!
Well Johnson was not in the shape to last in such a fight at 37 though, His best shot was to try and take Willard out early which he failed of course. Johnson was not stupid. If he fought at the pace of say the Flynn fight at 37 years of age in that 45 round fight, the odds would favor the younger Willard. So he though all his marbles early to try to get Jess out of there. Willard does have a good chin though, and took Johnson's best shot. I really think in round 18??(Been a while) Johnson try one last effort to take Willard out, before he gas out. I don't think Johnson though the fight, the round before the ko, Willard got Johnson with a good body shot, and of the one punch knockout was when I believe Johnson felt it was a lost cause. I don't think he thrown the fight, or even saying he quit, but he had his limits at age 37 in the heat.
Every single name fighter (and he fought a lot) that fought Larry Holmes with the exception of Leon Spinks and Marvis Frazier that fought Larry Holmes (including the shell of Ali) lasted to the 10th round. Larry might very well win the fight, but it is not likely to be within 10 rounds.