I used to live in Columbia, MO and hitched thru KC a lot. I actually got straight rides from Columbia to Denver no less than three times, so it is possible to skip Kansas entirely. I did always walk down onto the shoulder of the interstate though. It may be harder / more dangerous to stop cars going 70 mph, but it was worth it for me to get seen by more traffic. I don’t have the patience to hang out on an on ramp, and those 700 mile rides are testament to the the shoulder’s effectiveness. Flying a sign helps with the long rides too.
Maki40 is right though: if you have to piece rides together thru Kansas, it’s best to stay off the interstate. Hitchhiking is prohibited on most of it, and the pigs are persistent.
I don’t remember any particular locations to thumb out of KC, but just look for a good long Highway shoulder on the periphery that you can get to on the city bus routes.