Not sure when Gone with the Wind was written - about 1950? - but it was long after the civil war. So if you make a film of the book, there is an argument fir reproducing the 1860s as imagined by 1950s America, as you might do King Lear in Elizabethan clothes though it's set in the Dark Ages.

An oddity about film music. Watching the credits at the end, there are always far more pieces listed than I remember hearing. Somehow most of it is buried in the background where I don't notice it but where the producer thinks it would contribute to the overall effect.