I believe Carter would have lost all but GA and AR in the South. I do believe he would have won back MI, IA, IL, and held OH, KY, and WV due to the labor movements in these states. The story of the campaign would be the Last Stand of Organized Labor. This stance would have kept the industrial Midwest in the Democratic column, and MAY have even won over New Jersey, which was full of suburbanites, but also full of union members.
A Reagan/Ford ticket in 1976 would have been an impossibility. It would not have been credible. Ford would not have been part of it, and Reagan would then have had to run on the record that Nixon and Ford had produced for the Republicans. How credible would that have been.
In many ways, Reagan was better off losing the nomination in 1976. I doubt he would have prevailed in 1976 had he become the nominee; he would have run strongly in the South, but he would have lost votes in places where the Republican electorate was not as ideologically conservative. I'm not certain that Reagan would have carried California at that time. (Reagan was not overly popular when he left office in 1975 and The Almanac of American Politics assessed Reagan as being "probably unelectable in California".) Carter lost California BECAUSE of his relative cultural conservatism; a regular Democrat like Birch Bayh would have carried California against Ford. 1980 was a long way off, and a LOT had to happen for Reagan to be electable nationally, let alone as popular as he became.