Before you point out he's a light heavyweight, he's a modern light heavyweight. Meaning he boils down to 175 and is actually 185-190 in the ring. This means he likely would have fought at heavyweight in the 1970s and before. But if we go from the 1920s HW division to the 1960s HW division, how does Kovalev do? Would he dethrone Dempsey? Does he outbox Max Baer and Primo Carnera? Does he make sure Joe Louis is never heard of? Does he make Marciano go 48-1?
Kovalev at his best was an elite light heavyweight, but I just don’t see him doing that great at heavyweight. Especially in the past where the heavyweights were more physical and Kovalev’s infighting skills leave a lot to be desired. Kovalev had no answer when Ward started roughing him up on the inside, so I can only imagine what guys like Jeffries, Jack Johnson, Dempsey, Foreman, etc… would do to him.
In the 50s he'd do great. But the 50s is the smallest least powerful HW era. Most great LHWs in other eras could do fine in that HW era. Canelo could do great. Earlier? I don't know.
Foreman wasn’t an inside fighter… one of the most incompetent guys I’ve seen there, but yeah you’re right he’d need to have his clinch on point.