I know this is a prime for prime question but it is hard to locate both men's primes. Toney was either dead at the weight or threw his prime away - when he was humiliated by Jones he was considered in hs prime for example.
As to Joe's prime. His physical prime was no doubt when he beat Sheika. His best performances came when he was about 34, when he got his biggest wins. (If not fo this gap, I think he would have become a better fighter.) Soon he'll likely have scalps named Hopkins AND Jones on his record, so it's not a stretch to add Toney.
Toney has a right hand that would trouble Joe badly. A quality, accurate array of punches. But also a tenency to hang back, a lack of assertiveness.
Joe can definitely win big fights, BUT it has to be in the right place or at the right time. A prime Toney could have an off night, or whatever, and be delivered to Joe, who gets the win. However, Joe is sheltered, there are big gaps in his game against a counter puncher, and a lot of things that could go wrong if he fought on other ppl's terms.
Because of that, you'd have to say, if Calzaghe & Toney's careers had some overlap at the same weight, that Calzaghe likely would not entertain the notion of fighting Toney at his best. Based on what we've seen of his successful career at su[er middle and light heavy.