This would be an excellent fight. First thought is McCallum is a bit too crafty for GGG and earns a close decision.
I'd say GGG loses to all of the main four from the early 90s. McCallum would bash GGG's headback w counter-jabs and work the body. I think GGG would resort to a jab machine like he did vs Canelo, and he'd lose because of it.
McCallum was craftier and more versatile at 160 than he had been at 154, but he lost some of his firepower with the move up in weight and he developed a tendency to fade in the late rounds. I think McCallum probably gets off to an early lead but G surges on late to narrow the gap and perhaps pull it out.
