Mark,
Two very commonly overlooked aids for the VFR pilot (or the light airplane pilot) are road maps and enroute low altitude IFR charts.
Road maps, especially in mountainous terrain, will give you a quick reference to lower terrain and passes. Highways don't go over mountain peaks; they go between mountains. They're a good reference when planning routes, to be used in conjunction with a sectional or a WAC.
Low Altitude enroute charts give quick references to minimum altitudes along specific routes between navaids, as well as frequencies for use in an emergency. (A low altitude IFR chart may provide the information you need when sitting on a hillside after a forced landing, trying to call for help).
From portand, head toward Boise, then down through Salt Lake City, Grand Junction, and over to Denver. If there is weather, or you don't want to go over the higher country, go south to New Mexico and over. Alternately, you can go from Boise to Rock Springs, with resonably low enroute terrain there. Eastbound from there is straightforward, with relatively flat terrain that keeps getting flatter and lower the farther east you go.
As always, watch your weather, and be prepared to stop and wait it out, if need be. Remember, there are no flights that must be made. Dress warmly, carry some extra supplies (and cash). We're moving into spring time, but many areas along that route are very inhospitable, even for months to come if you need to put the airplane down somewhere. Be prepared. Good luck!!