Joshua knocks all of these fighters including Marciano out in 1 round. Because he is true modern sized heavyweight and neither of these fighters are .
Can we at least wait a bit! What we know so far is he is a super heavyweight with skills, power, and speed. Other questions needs to be answered. 1 ) Let's see to Joshua beat someone ranked 2x in top ten when he fought them. 2 ) Let's see if he can take a good punch. In the Amateurs, he did not. 3 ) Let's see him go 12 rounds at a pace where his opponent at least presses him a little. 4 ) Lets see if he can keep in mental and physical shape/ If he can do all of the above without losing for 5 years then I see at 48-1 or 49-0 vs the same guys Rocky fought. Marciano's cooption in title matches was decent. Better than say Dempsey's or Johnson's, so if you think Joshua would roll though Marciano's completion, you'd have to say the same for Johnson and Dempsey.
No need to wait. The OP question was Joshua as he is NOW against Marciano's opponents. Not when he has more experience, proven his chin or stamina, not when he has beaten numerous top tenners,or in five years time but Now. If people thinks that he's won't do well or lose against Marciano's opponents now because he's unproven or currently lacks experience and longevity then just say so. It's your opinion and you are entitled to say it. Personally I think even at this relatively early stage of his career, I would be surprised if he lost to any of them.
Joshua was TKO'd floored and stunned in a short amateur career. Do chins get that much better in the pros? I'm not sure how good he is in terms of durability, stamina for the later rounds, or facing smaller but very skilled opponents ( the ones Marciano fought ), so I'll fence sit until I know more.