I would include in the overall Midwest definition those parts of Montana, Wyoming, and Colorado east of the Front Range, as well as western Pennsylvania and northern West Virginia due to the similarities in economy, population, and language to their Midwestern neighbors. I would exclude Oklahoma due to the predominantly Southern influence on that state.
Certain Midwestern distinctives, like the hard "r", may reflect the Germanic roots of much of the region's population. Regarding Gone With the Wind, it is worthy to note that Clark Gable, a native of Cadiz, Ohio, and partially of German ancestry, did not even attempt to sound Southern. During the Golden Age of movies, a large number of actors from mid-America did not try to sound Southern, even when their roles called for a drawl: John Wayne (Iowa and California); James Stewart (western Pennsylvania); Gary Cooper (Montana). OTOH, in the recent movie Cold Mountain, only one of the lead actors (Renee Zellweger) was from the South. Nicole Kidman (Australian), Donald Sutherland (Nova Scotian), and Jude Law (English) all delivered competent Southern accents.
When we were in Ireland we were asked many times if we were from Australia. I live in South Carolina and have a thick SC accent that I don't try to mask at all.
I always thought Australians sounded British but to the Irish there is a definite distinction, and they mistook our Southern accents as Australian. Since the Irish/Scots heavily settled the southern US and Australia, I'm sure there is some similarities in the two dialects.