I don't know that Holyfield would have done well in Marciano's era. He got knocked out by Toney, who handled him very easily, very one-sided fight. Yes, Holyfield was 40 but Toney was 35, and a fat middleweight to boot. After the fight Evander said he would never have beaten Toney because he couldn't figure out how to hit him. Bill Miller, he trained Toney from early on, was a disciple of the Ezzard Charles/Archie Moore/Walcott style of fighting.
Or look at it this way...put a 40 year old Evander (ko'd by Toney, easily) against 40 year old Archie Moore, who dropped Marciano and gave him a tough fight, and then went 37-3-1, defending the LHW title and fighting for the HW title. Or a 36 year old Moore against a 36 year old Ali, or the Walcott that fought Marciano against the Ali that fought Berbick. Same age.
I definitely have a bias towards old fighters because they were better fighters. The trainers were better and they were able to fight more often, which I am sure todays fighters would do, if there were opportunity to fight twice a month.