Hubert Humphrey died of bladder cancer, which went terminal in the late 70s and was one of the reasons he didn't run in 1976. He died in 1978.
Reading over some things, I came across that he started to notice blood in his urine around the 1968 election, but didn't get it check out because he didn't want to hurt his Presidential campaign by undergoing an aggressive medical evaluation. By the time, some 9 years later, that they did finally diagnose it and begin radiation therapy, it was too far advance and Hubert was terminal.
So what if Hubert Humphrey didn't have that trepidation (maybe RFK got the nomination and he didn't have to run, maybe Humphrey isn't VP, whatever else is needed for the scenario) and does get things diagnosed earlier and begins treatment earlier?