To me, one of the things that make a Southern Mt. gun is the crescent butt piece. So, if you change that, I'm not sure you'd have a "Southern Mt." gun. Then again, I'm no expert on Southern Mt. guns...
If you're not really fond of the crescent butt piece, there's plenty of other schools that have a straighter, wider butt. Lancaster's come to mind, as well as some earlier Virginia guns, as well as others.
I like a 46" barrel. I've built two rifles with the swamped barrel Rice offers and thought they both handled, and held, very well. I was concerned about the length, but found they were very well balanced guns. Plus, there's just something about that long barrel that to me, speaks Longrifle! I did them both in .50 cal primarily to keep the weight down, but I don't know if there would be a big difference between the .50 and .45. Good luck, and I'm sure you'll be happy with a 46" barrel.
Ed