Love the Rock, but Cruiserweight Holyfield might be THE best example of a "Brick Shithouse" in boxing. Marciano would not be able to deal with that level of activity. Prime Holyfield had an iron chin and video game cheat level recovery.
Although I believe Marciano is a great boxer with underestimated defense, his style would not perform well at the elite level in recent generations. Size aside, he'd need to tighten up his defense and work on his mobility. Prime Holyfield was a great example of expert technique complimenting a fighters physical attributes.