I have always regarded Foreman as possibly being the strongest heavyweight in boxing history. Just watch Foreman's bout against the exceptionally strong and durable George Chuvalo. Foreman literally manhandles Chuvalo for three rounds before the referee stops the fight. Very, very few heavyweights in history could have walked through Chuvalo like that.
Also check out the way Foreman treats big, strong heavyweights like Joe Frazier and Ken Norton as if they were beach balls. Incredible.
My tentative guess is that John L. Sullivan and Jim Jeffries were not nearly as strong as Foreman was. Sullivan especially must have been weaker than Foreman, as John L. was only 5'10 1/2", 188lbs. to George's 6'3", 225lbs. (prime).