I would suspect that, even had Nixon won such a sweeping victory - (which would involve a national result well over 2/3rds of the vote and a change in Civil Rights Policy vis-a-vis the GOP and the African-American community sufficient to keep that community voting GOP as they had up until at least 1932 {and a large proportion until 1960} and at the same time sufficiently acceptable to the South) - that one elector, at least, would have voted for Agnew or Jesus Christ in order to deny Nixon a unanimous vote, in order to keep such a result limited to the Father of the Country, George Washington. Such a result occurred in 1816, when James Madison was denied a unanimous result in the Electoral College after the Federalist Party was eliminated as a political force (due to a treasonous taint likely).
In order to achieve it, as has been pointed out, you first have to take Massachusetts, which would reasonably be acheived by limiting the vote to 21 year olds in a state with many residential college voters.
DC would need a change in African-American voting patterns as even then (before White / Suburban flight) they were half the vote.
You would also need to stop the faithless vote historically made.
You would finally need to overcome the tradition of non-unanimity that honors the pre-Partisan era of Washington.