Because it's been years since I've seen it, but I recall that it sure looked like it was shot in California's Sierra Nevada Mountains. The Kansas Raiders were running up and down pine-covered hillsides, with big outcrops of white granite all around. More mountains were visible in the distance.
I haven't been to Kansas since I was a child, but isn't it on the flat side? And lacking in pine forests?
You are right. The movie was definitely shot in a far western setting. There was rocky mountainous terrain with western type pines. I have been to the Kansas City area, and it is wooded, rather flat, and definitely more midwestern in appearance. I don't know what western Kansas is like, but eastern Kansas definitely does not look at all like the obviously California filmed territory.
This is not the first time this has happened. For example, in the pilot episode of Little House On The Prairie, within a few days walk of southwestern Minnesota the children are lost in an obviously western, rocky wilderness terrain. I have also heard people comment that you can see the Sierra Nevadas in the far background. The Dukes Of Hazzard is supposed to be in Georgia, but you can tell from the car chases that it is actually California.
I hope noone thinks Universal would gather up maybe 200 cast and crew members and trek to Kansas to film an Audie Murphy B Western. "Hey, in the interest of accuracy, let's add maybe $250,000 to the budget!"
May I bone your kipper, Mademoiselle?