I've done a couple of these flights in 500's....and prefer Gordy's routing. If you cannot get over the Mountains east of Seattle....head down to Portland and go up the Columbia River (mind the wires) and route to The Dalles, Pendleton, La Grande, Baker, Boise, Pocatella, and into Wyoming. This time of the year you will have a good tailwind I would assume....pick a weather window if you have that luxury and do it in some severe clear and enjoy a beautiful trip.
I preferred to stop at smaller airports and avoided large airports....the refuel is quicker, the folks are nicer usually, and makes the trip much quicker as a result. Lots of times the fuel truck was ready for me when I arrived if I called on Unicom.
In Washington, Oregon, Idaho, Utah, and Wyoming....IFR works good....I-5, I-84, I-80......with a detour along US-30 north and northeast of SLC.
