It would be a good matchup. Williams was a better fighter than Luis Firpo, IMO. Dempsey came in wide open for Firpo and got tagged hard and often. Jack would have to show Cleveland a lot more respect and care against him than he gave Firpo or Willard. Cleve would not be as easy to hit as those guys. Jack would have to show the savvy he used against Brennan and the stamina he showed against Brennan and Gibbons. I think Jack would likely win, but he would have to be at his best.
William's resume at the highest level is pretty thin. I get it that he was a dangerous fighter, but Dempsey would be the betting favorite.