I'm not sure about the other neutral states, but Missouri would still pretty much be a neutral state if this were to occur. Mostly because there is a lot of people who live in the cities and vote liberal, so they would go with the north. The rest of the state, would go toward the south. Those tensions are still present today, as I see it in my own small community back home.
However, the majority of the number of states are still Republican states, they would side with Texas.