I can say that the world will end with confidence.
I can say with confidence that the world will end.
I know that people would avoid to say the first sentence, but it is highly unlikely that someone would interpret it as saying "with confidence, world will end". So here comes the question. It is possible to say both of those sentences, but I don't know why we can shift the "that clause". Does it work in the same way as "heavy noun phrase shift"? Or is it because this that clause is used as a noun being an object?
Here is my assumption.
3."I can say that the world will end."
4."I can say with confidence."
Both sentences work greatly without a that clause for the fourth sentence and without the prepositional phrase, "with confidence," for the third sentence. Thus I am assuming that changing the order of these two, that clause and prepositional phrase, is possible since we can delete one of them and still make sense, which makes changing order of those two do not matter at all. Is my assumption correct? If not, please tell me why it is grammatically and idiomatically correct to shift it around.