Don't think anyone would pick Valuev despite the huge size difference but just how difficult a fight could he has made it for Rocky?
One of the things you notice about Valueu was how small his fists were in comparision to his overall size. Must be why he could never punch his weight otherwise he would have a far better puncher than he was.
Probably a pretty ugly fight to watch, with Marciano taking a decision based on activity. It might turn out that Valuevs chin was not as good as we thought of course.
If the Frenchman Bergeron could hurt and stagger Valuev, I needn't go into too much detail as what Rocky,....despite the great size disparity...would do to him.