There is a CBS/New York Times exit poll cited on Wikipedia which has "White South" going to Ford 52-46, although no link is provided.
Anyway, Carter won the former Confederacy by slightly more than 9%, and the Census-defined South by slightly less than 9%.
Considering the plausible ethnoracial demographics of the Southern electorate in 1976, I don't know why it should be unreasonable that Carter lost the non-Hispanic White vote (especially using the "former Confederacy" definition).
Right now I don't have the time and resources for more detailed or complex calculations.
Fair enough. It is the county maps that make it deceiving. But I guess the same could be said today about Democratic states that only have a half dozen or so counties vote for the winner. 😛