Burt is a primary source. He has seen with his own eyes legends from the 1940's, and he's telling us that they were just as, if not better than modern fighters. doesn't get any better than that.
The gulf in class between Willard and Dempsey was just too large. It wasn't apparent at the time, maybe, but its true. I don't see Willard ever winning that fight. Too slow, not enough footspeed, not enough skill ... you can't win a fight on heart and chin alone, not against Jack, anyway. Dempsey is one of those guys whose actual ability is much better than their resume.