It's not just the nutrition and training stuff, but the nature of boxing back then. None of those guys had the luxury of preserving their body and taking full training camps for every fight.. 38 in Walcotts case was clearly very old.

I have no idea how Marciano would have handled Michael Spinks, they both could have easily been great cruiserweights and that probably would have been a hell of a fight there. I don't think the Tyson fight has that much bearing, but I also feel Iron Mike would have pummeled and stopped Marciano.