Rockefeller was far too old to be allowed to be active enough to go around outside. He himself was a very powerful man in both personality and build, but I don't see he Secret Service letting such an old man go around enough to be killed, particularly since he would have just come into office after Ford was killed.
If Rockefeller lived, I doubt Reagan challenges him. Barry Goldwater, Jr., or Ron Paul might, but he'd sweep the primaries with a sympathy vote. If you insist on the extremely unlikely scenario of his death, Albert would probably pick either Jackson, Udall, McCarthy, or Glenn. Udall or McCarthy would appease his base, while Glenn unites the party and Jackson adds charisma. By the end of 1975, Jackson is at State and Symington at Defense.
340: President Carl Albert(D-MA)/Sen. John Glenn(D-OH) - 52.5%
198: Fmr. Gov. Ronald Reagan(R-CA)/Sen. Christopher Bond(R-MO) - 44.4%
Fmr. Sen. Eugene McCarthy(I-MN)/Rep. Pete McCloskey(I-CA) - 2.2%