Another alternative to the Interstates...
US 36 across Kansas to St. Francis. Then take KS 27 north to Haigler NE, and pick up US 34 there. Then take US 34 (you'll briefly get on I-76 at Brush to Fort Morgan to Wiggins), all the way west to I-25 at Loveland. I-25 north to Fort Collins and on to Cheyenne.
This route, you miss the Denver metro area and the parking lot formerly known as I-25 between Denver and Fort Collins. Once you get north of exit 269 on I-25, traffic settles down considerably.