I think that Rockefeller would have helped Goldwater carry some of the traditionally Republican states in the Northeast and Midwest, but Rockefeller would have turned off voters in the South due to his strong support for civil rights, thus causing the National States Rights ticket of John Kasper and J. B. Stoner to win several states in the Deep South.
Here is what the map would look like:
Presdident Lyndon Johnson (D-TX)/Senator Hubert Humphrey (D-MN): 464 Electoral VotesSenator Barry Goldwater (R-AZ)/Governor Nelson Rockefeller (R-NY): 39 Electoral VotesMr. John Kasper (National States Rights-NY)/Mr. J. B. Stoner (National States Rights-GA): 35 Electoral Votes