Your route seems nice, I would love some more details !
We pretty much used all Adventure Cycling Routes and maps (TransAm: yorktown to missoula, Great Parks North, section 2: for Missoula to Whitefish, then Northern Tier: from Whitefish to Rockport). Rockport is where we turned off route on Rt 530 to head southwest towards Seattle, and when I tried to create a "map my ride" route of it for you, it wouldn't let me pass through Oso, site of the massive mudslide tragedy you may have heard about. The road is currently buried under acres of mud, and I'm not sure when it may be cleared (hopefully by this summer). You may have to go further due west on the NT before heading south, which would be a shame because I recall 530 being a pleasant ride, with the town of Darrington having a great diner and a clean, cheap motel. 530 takes you into Arlington, and the jct. of the Centennial rail trail (paved), which you can take for 25 miles or so due south. Depending on where in the Seattle metropolis you wish to go, the on-line tool "map my ride" when used in cycling mode will show you other bike trails (seattle is a very cycle-friendly city) and streets with cycling lanes that you can piece together to cover the last 30 or so miles. We headed west around lake washington on the Burke-Gilman trail (another very nice paved railtrail) which heads to the University District.