As much as I love McCallum , Joe's size, Handspeed and volume punching would be a problem. I think Joe wins via decision. McCallum was in his mid to late 30's at 168.
McCallum is the greater fighter p4p. Assuming we're talking about a prime Calzaghe, then at 168lbs with day before weign in, he beats any version of McCallum, imo.