Done the trip from Denver to Portland many times. No good way to avoid mountians and snow on roads. I-25 to Cheyenne, is fine, then Cheyenne to Provo is up and down crossing the continual divide at 8000 feet, then north from Provo to Idaho through Snowville. Does the name give you a hint? Then through Boise into Oregon and the blue mountains and more passes. Once you get to pendalton, Oregon you drop in elevation but then you hit the Columbia gorge. They close it often during winter storms. The trip is a beautiful trip except for the stretch between Laramie, wy and Evanston,wy.
I think the Panama Canal idea is the best option.....
