What makes people so sure it would be by KO? Calzaghe didn't look great against hopkins, but it was an immense clash of styles and isn't necessarily an indication of how he would fight against spinks, nor doe it indicate he would be ko'd?
First Calzaghe did absolutely nothing at the weight and went down twice in two fights. Spinks is known as one of the best LHW in history. He was undisputed champion in the best LHW era in history and won most of his fights by knockout. He went up to heavyweight to challenge a great heavyweight champion in Larry Holmes who was 48-0, won two close fights against him and KOed Gerry Cooney. He had an awkward style but was a pretty good boxer and could hit. Calzaghe never faced someone like Spinks.
I think the reason that folks are skeptical about Calzaghe surviving against Spinks, is that in the only two fights that he partook in at lightheavyweight, he was matched against two 40+ year old men - neither of whom were original members of that division and neither had quite the power that Spinks had at that weight, yet both managed to floor Joe early... Although I have all the respect in the world for Joe Calzaghe, a 25 year old Michael Spinks who was a natural lightheavyweight, and one of the very best by all standards would be an extremely dangerous proposition for a 37 year old Calzaghe. I think its possible that Joe would have his moments of good boxing and might steal a few rounds from the Jinx, but the ods don't ad up much in his favor..
You have to go with Spinks by stoppage. Calzaghe is difficult to match with the greats at 175. Only fought there at the end of his career when physically he was in decline. Pre the creation of the SMW division he may have been a more natural LHW than MW though. Despite this to me he looked softer at LHW, but would be emaciated at 160. 168, despite his oft claimed struggles with weight seems a good fit for him.