President Rockefeller would have won with ease. An incumbent who got in office by an assassin's bullet just over a year before the election is very hard to beat, and I have little doubt Rockefeller would have governed pretty effectively. He may have bled support from conservatives, but he would have attracted a lot of crossover appeal and support from the African American community.
However, Rockefeller would have died in office himself, as he passed away in 1979. I think his successor would have lost in 1980 then.
✓ President Nelson A. Rockefeller (R-NY)/Vice President Howard Baker (R-TN): 372 EV. (52.86%)Governor James E. Carter (D-GA)/Senator Walter F. Mondale (D-MN): 166 EV. (46.19%)