If you want RFK to be POTUS later than in OTL a much simpler and more plausible way to do it is to have him not assassinated--and lose the 1968 Democratic nomination. The notion that victory in the California primary guaranteed RFK the nomination if he lived is largely based on a misunderstanding of how the nomination process worked in 1968--primaries were not as decisive as they are nowadays. While RFK and McCarthy were knocking each other out in the primaries, Humphrey was quietly accumulating enough delegates to put himself within hailing distance of a convention majority. I know some people think that Mayor Daley and others would have switched to RFK if he had lived, but I am skeptical about that; see my post at
https://www.alternatehistory.com/fo...ted-rfk-if-he-had-lived.443752/#post-17022521
If RFK is not seriously wounded and gets the nomination, I think it unlikely he would turn it down--it would go against his self-image of courage. What is much more plausible is that he won't get the nomination.
(Incidentally, I doubt that RFK or any other Democrat would beat Nixon in 1972 anyway--I think 1976 would be a much more promising year for RFK--but that's another matter.)