Joe fought mostly at 168 in the era of day-before weigh-ins and often talked about the physical drain it took, so most likely he'd be at 175 under the old weigh-in rules in the early 80s (even if there were a weight class limit of 168 around back then). How does he do in the early 80s at light-heavyweight against the likes of Spinks, the 3 Muslim champs, and others?
Calz would probably beat some of the titlists such as Johnson and Stewart (whom a green Virgil Hill KOed) and probably lose to Qawi and Spinks. Saad and Mustafa Muhammad are tough fights to call either way.
Michael Spinks knocks Calzaghe out, no question. A horrible stylistic fight for Calzaghe. I'd take Saad Muhammad to stop Calzaghe somewhere in the middle rounds. Dwight Muhammad Qawi beats him as well.
Spinks would stop Calzaghe at 175. He is too big and too fast. His ability to shoot hard shots from the outside would spell the end of Calzaghe. Calzaghe straightens up far too much and his inside swarming would actually be a detriment to him in this fight.
