To play the game, I am with those who say 168 would be his peak weight. But to play Devil's Advocate; I would argue in this day and age, with the way people train and a diet a hundred years on, I could easily see a 2021 Greb walking around at 190/200 and bemoaning the need to get down to 175.
Greb is a classic example of somebody who did not have a suitable weight class for him. He struggled to make middleweight, but he was a midget at light heavyweight. Even so, he obviously did alright for himself at light heavy, and there have been many times, when the best man under 175 has been a natural super middle.