Why not merge North Dakota into these states? Then it can be East and West Dakota, divided by the Missouri River.
Past Kingpoleon:
Atlas took your post seriously. STOP. Advise time travel preventing posting that. STOP. Do not attempt further contact. STOP.
- Future Kingpoleon
How about using the Mississippi as an eastern boundary, and dividing the area to the west using the Minnesota&Continental Divide, Missouri, Arkansas, Red, Brazos, and Rio Grande as boundaries. Eastern Minnesota and Louisiana would be added to Wisconsin and Mississippi.