My wife and I used Highway 20 from Newport, OR to Boston MA. Highway 20 and 26 share the same road in a number of areas. I suspect that is the route you are looking at. I'm not sure where to head north to hit Madison, but Dubuque, IA seems like a good point with Madison not too far north. Highway 20 is a great route. It is the longest contiguous coast to coast highway in the US. Also there are not many bikers on that route so you are still a novelty, and people are pretty open. Some improvisation is necessary, but we really enjoyed the ride.

I live right in the foothills about 45 miles west of Santiam Pass. Depending on your timing, I'd be glad to ferry you over the pass. We live very close to Highway 20, which is one of the reasons we picked it as a route.

Take a look at our blog site in 2007 to get a feel for the route.
Doug and Donna's LiveStrong Adventures: June 30 - July 2: Sweet Home to Bend