Twain wasn't Southern, was he? He was born and raised in Missouri, and spent much of his life in Nevada, California, New York, Connecticut, and elsewhere. Missouri was considered part of the slave-owning South in the 19th century, but it's more Midwestern, really, isn't it?
Well as I mentioned in my previous post, MO has accents of it's own, often referred to as an "ozark twang." Not sure if that's what the actor was going for or not, but it was NOT a southern accent.