Marciano would be in over his head literally and figuratively against George,too slow ,too small ,it would be similar to a lightweight challenging a middle.Dempsey has a better chance because he is more elusive,and quicker into range , and has more reach.Plus we know he has a good track record against big men , but it's a tall [no pun intended], order, even for him.
I see prime Foreman beating both. Maybe Marciano loses by KO2 and Dempsey by KO9. Older Foreman beats Marciano KO7, and Dempsey... I really, really don't know. Just my thoughts.
And Quarry basically put Shavers on Q-Street the first time he really tagged Ernie. It's the Heavies....where anything can happen.