What about Rockefeller/Goldwater vs. JFK/LBJ? I'm thinking Rocky gets the nomination and picks Goldwater as VP in a show of party unity and to help him down south where Goldwater is more favorably polling. The resultant race would undoubtedly be close, like 2000 level close.
If I did this I might have Goldwater deliver a few Goldwaterisms in debates and on the trail to the chagrin and detriment of Rockefeller to enhance the suspense and establish what'll end up being a rocky relationship (no pun intended) if/when Rockefeller wins. Or I could have JFK win, idk. In any case I think that scenario would be believable and interesting to write/read.