That the 25th Amendment does not include all the things the Succession Act mentions does not mean it moots those on which it is silent. ISTM that after the Amendment passed what would have been in order would have been to pass a new Succession Act that derives its authority from the newly amended constitutional language and that covered more possibilities.
As it stands now, the amended constitutional language deals only with the Presidency and VicePresidency under the assumption of certain scenarios:
(A) The traditional, happened-several-times-before ones of permanent vacancy of the sitting President wherein sitting VP ascends to President; or when President is still in office, but permanent vacancy of a sitting VP happens.
Before the passage of the amendment, when there was a VP succession or a midterm vacancy of the VP, the rest of the term simply passed with no VP. This was done away with and put into effect for the Nixon/Agnew/Ford/Rockefeller sequence, but is important when looking at the situation once you start going down the rest of the order. More on that later.
(B) The temporary incapacity of the President. Before, this was entirely unmentioned and people were concerned about for example who was really calling the shots after Wilson had his stroke. The need was seen to clarify that. Notice nothing was said of the temporary incapacity of the VP, confirming by omission the utter functional innocuousness of that office in normal times.
The text of the 25th Amendment does *not *enter into any discussion however of what happens in the situation of absence of both or of what is the scope of the Acting Presidency in cases other than incapacity of POTUS. What happens in the absence of both officials remains entirely governed by statute.
Meanwhile, the 1947 Succession Act was written at a time when in the case of the death/removal/resignation of a sitting VP, the office of VP would have been allowed to remain vacant for the rest of the term.
As pointed out, the Act’s “acting” provisions contemplate the possibility of the specific living person or persons higher up in the chain being temporarily disabled, or prevented from taking office on the mandated date, but with the possibility of that situation being corrected, upon which the presidential powers would go back up to the highest-ranking and qualified (the “bumping” provision). However… since when that law was passed, if the sitting VP was removed/died/resigned, he would NOT have been replaced, in a subsequent vacancy of the President the Speaker would become Acting President implicitly for the rest of the term since the only way to refill those offices was the next election.
Apparently Truman favored a provision that if the vacancies happened before the midterm election, you’d have a special election just to finish the term (as happens with many Senators). That did not happen.
IMO the Succession Act *should *have been amended after '68 to acknowledge that now the Constitution provides for filling the vacancy as VP, and to specify explicitly whether or not that means the #2 and lower persons in the chain will accede to Acting Presidency only as caretaker Head of Government until a new VP is confirmed and then automatically becomes the new President by Right .