Reagan/Rockefeller (R) 305
Humphrey/Carter (D) 233
Reagan was always an "includer" and including Nelson Rockefeller would have brought about unity for the GOP.
Carter, after nit-picking and criticizing McGovern prior to the 1972 DNC, lobbied to be his VP. He was the guy that actually wanted the VP pick, but McGovern wouldn't have him. I believe he'd have done the same to HHH, and Humphrey would have picked Carter, who could be persistent and would have offered the illusion of being able to carry the South.
I'm pretty sure that given this scenario, the nation would have had a bad case of Johnson fatigue.