Well here's a list of fighters who don't make the grade. Mike Tyson. Roberto Duran. Sonny Liston. Sam Langford. Joe Gans. Willie Pep. Barbados Joe Walcott. All quit.
Marciano was one of the most fit fighters I have ever seen, today they would change his methods and he would be heavier because of muscle training. I see guys today at 240 and 5"11 but I think Marciano would keep it down to about 220 or so and with weights and supplements and look something like Mike Tyson thing is Marciano trained very hard to support his mental state which may have been the strongest thing about him, his mind was intelligent and he had great belief in himself and refused to lose. I think today at heavyweight they allow more weight because of the 3 round shortage in champion contests. It is a lot harder fighting 3 rounds after 12 rounds if you are carrying too much weight and especially at a Marciano pace, it would be impossible for these modern sluggards