So you are totally dismissing that weight divisions exist for a reason? This isn't who is the best P4P this is who would win in a fight when they are competing in the same weight division. You can't compare small HW's of the past with today's behemoth HW's. Usyk was a CW arguably one of the best all time who is now competing at HW and people are questioning if he can handle the bigger HW's. But for some reason you think Marciano who was even smaller would handle it with no problems.
I have never stated whether Marciano could have beaten modern day HW. What I can say is that fighting people around his own weight, he had tougher opponents and a better record than Mike Tyson.