Even with his gaffes, Ford almost defeated a more credible Democratic candidate IRL than Brown. Besides, Brown had only been serving as Governor of California since 1975, hardly the stuff a credible Presidential nominee is made of.
VP picks are interesting, but Buckley would not help Ford carry New York, and I do not believe that 1976 America was ready for a black Vice President, as commendable and impressive as his military record was.
This map is the very best case scenario for Brown, given the scenario you have outlined, major gaffes by Ford and a flawless campaign by Brown. In reality, had Brown been the nominee, he would not have done this well. He was simply too inexperienced to be taken too seriously by most of the electorate.
Ford/Buckley 274
Brown/James 264