Google Maps is the first site I go to when planning a route, but for something this long, I'd head over to Adventure Cycling and order some of their maps. Their routes have an excellent reputation in the touring community, and their booklets are well worth the money; they're cycling-specific, and come with notes telling you about bike shops, bike-friendly businesses, trails, and so on. It looks like combining part of their Transamerica Trail with their Western Express route might work for you.

Once you have a rough route, you might want to learn from people who've cycled there before you. Crazy Guy on a Bike has tons of tour journals that went through San Francisco. (The ones around Pittsburgh are mostly tours from Pittsburgh to DC, a very popular route.)

Once you have a route... keep in mind that once you hit the road, be prepared to toss all your planning if you think of a better way to do the tour!

Have you toured before? If not, I highly recommend riding some shorter tours first to "shake down" any issues or gaps in your experience when you're close to home.