People have said bumping bodies with Marciano hurt. That should tell you how tough, and physical he was. I've never heard that about another fighter. As for the Ali reference it was H2H so when people bring up Ali it's really irrelevant.
As for Foreman and Marciano, they both have SICK power. Foreman was a big dude himself, but Marciano had that with his frame and small body. P4P Marciano had some gifts man. He also had a 2 handed attack unlike Frazier who would limit himself from time to time