It would probably require changing up the Dakotas a bit, but not too much. The Platte runs at approximately the 41st parallel north through Nebraska, so you could have Congress divide that area into thirds like they did with Colorado, Wyoming, and Montana instead of fourths like OTL.
This actually works really well. My next question would be does this affect anything by 2013? I think for Kansas it doesn't It would have more people, but not too many. Also, the only large city south of the Platte is Lincoln, and since it wouldn't be the state capital or the site of the University, or any other state institutions of that nature, it would be lucky to have 30,000-40,000 people by 2013. It would be an economic center of ATL northeast Kansas but that's it, and also it wouldn't be called Lincoln since before it became the capital it was called Lancaster (Idk if there is a lancaster Kansas in OTL).
As for Nebraska (I'll stick with the area north of the platte being called Nebraska and adding 2/3's of South Dakota, I think Omaha develops much as in OTL due to the Railroad, as well as being the largest city in the state. It won't keep the capitol though. I could see it being moved to Yankton since it was the capital in OTL Dakota Territory. It would make more sense anyway since having the Capital all the way in Omaha would not exactly be smart and potentially lead to separating. At least Yankton would be somewhat in the middle. This means that Yankton, being the Capital might be a decent sized city. Not huge by maybe between 50,000-60,000. Also, whatever city gets the state university will probably decent sized as well. Vermillion might work, though an institution further west would probably be built at some point. The only city that would be different in OTL would be Sioux Falls. Since it would only be a small city on the Minnesota border, it might only have about 50,000 people if that. The rest of Nebraska and SD develops as in OTL including the Black Hills. The only problem I see ATL Nebraska having is that the Indian reservations would make it somewhat poor. However it would have a good farm economy and the Black Hills tourism economy and even some mining.
The land north of Nebraska would be simply called Dakota and would have its capital at Bismarck as in OTL. It wouldn't be too different from today's North Dakota except for a slightly larger population.