You migth consider the ROAM route. Any route good enough for velomobiles should be great on a bike. Start was PDX and goes directly through the middle of MN. A lot of planning went into this route.
Search "ROAM" "Roll Over America" on the web and on BROL
http://carfreechallenge.blogspot.com...ica-route.html
http://www.bentrideronline.com/messa...ad.php?t=78496
and many more details on the web.