At the risk of putting you on some lesser travelled roads depending on time you may want to consider taking Highway 2 across Wa through Idaho and into MT. Take a day, drive over the Going to the Sun Highway in Glacier National Park then cut down to Great Falls then to US 89 through White Sulphur Springs down to the freeway, the scenery is gorgeous, even better than what you'll see on the freeway alone but these roads, while in good shape, don't have the volume of traffic that the freeway obviously does. For what it's worth too, I'd cut across SE Montana to Belle Fourche on I-90 then US 212, you get to see Devils Tower but it is a remote part of the country, I'd be surprised if they get cell service out there.