RJJ would have to box a perfect fight. Marciano can fight like crap and get lucky once. More or less sums it up. That said, if there's one guy would COULD fight a perfect fight it would be RJJ at his peak. But we're asking him to climb a mountain here. Marciano was relentless and wouldn't fight at a nice lesuirly pace like Ruiz. I'd have to favour Rocky in this one. I think it would look like Canelo / Khan. RJJ would clown him for half of the fight, but slowing down just a little every succeeding round. The end would be abrupt and messy when Marciano caught up with him. But if RJJ could keep his legs and distance for the entire fight he'd come out with a reputation second to none.