I think people overrate how much being a "Southern Baptist" would have put off non-Baptist voters in that era. Evangelical Protestantism has practically militarized since then. For example, I remember reading an interesting article that the assertion on behalf of fundamentalists that Christian tradition maintained that the Earth was only a few thousand years old and other interpretations were "backtracking" was originally only associated with the Seventh Day Adventists, yet Evangelicals adopted it militantly in the 1980s and beyond. Just one example, but I think as the nation has become more secularized, so too have Evangelicals become more radical and intolerant of more traditional Christian diversity in thought.
Evangelical schools in the 70s were already teaching that the earth was created in 4004 BC, based on Ussher's calculations. I know this from personal experience and I would qualify it as "militantly", at least in the south, though I dont know about the 60s or earlier.
Back to the original post, I think New England was in general suspicious of southern evangelicals, who had not been involved in national politics before. In 1976 they had to swallow their concerns if they didnt want to vote R, but they had an out in 1980 with Anderson, and Anderson did best in the northeast.
