Bad analogy. Greb was nothing like Marciano at all. He was much quicker, harder to catch, and didnt have a lot of power. Few heavies had the footspeed to catch up to Tunney. Greb did, but then again he was a middle / super middle.
Better may have been going a bit far, though I believe him to have been at least on par with all of them, especially head to head at heavyweight. I do believe him to have been, in his prime, better than the versions of Charles, Moore and Walcott that Rocky beat, and, like you said, a more difficult style for Rocky than any of them.
Important to remember that Tunney was on the ascendency when he quit. His trainer and sparring partners said he was hitting way harder and was much better than in preparation for either Dempsey fight. I think this version of Tunney is slightly better than Charles and certainly superior to Walcott. All said, it's really hard to pick against the Rock. i do it here by the slightest margin and with allowance to reverse myself at any point in the future.
Over 10 I'd definitely go with Tunney. Over 15 it's a much closer fight. I think it's a 50-50 contest...
I don't know...... An even match between boxer vs. slugger.... Okay, with a gun to my head..... Tunney wins a 10 rounder with Marciano, but gets KO'd late in a 15 rounder by Marciano... Dual outcome... Peace.... MR.BILL