John L Sullivan in his formative fighting career was known to be able to brawl and maul with his strength, his conditioner later was wrestling champion Willian Muldoon.
Dwight Muhammad Qawi was very physically strong, he often beat up guys bigger and taller than him. James Scott was also very physically strong aswell as was Victor Galindez.
I think there are a few types of "strength" in boxing. There's literal raw strength, Foreman shoving guys around like they were nothing is one of the best examples of pure strength and the express intention to apply that strength. Foreman, at only 21 yo, hurling seasoned strongman Chuvalo half way across the ring never gets old. At least on appearances and to an extent as demonstrated, I'm sure Liston was very strong but he never tried to shove and maul as Foreman did. Even Ali shoved Liston back in a more physical than Liston ever practiced/reciprocated - even though Sonny could well have done same if inclined. A guy simply moving forward behind power shots, with a chin that is impervious to incoming artillery simply looks strong, and is literally strong for the pure intents and purposes of boxing. Without any shoving or mauling (in a very clean fight actually) Liston, taking and walking through Cleveland Williams' bombs, looked incredibly strong - great chin, great power, perfect ingredients for going forward and conquering. Up to a certain level of power, Joe Frazier too - he just kept coming and punching. Very "strong" fighter.