I used to live in Coeur d'Alene, ID, and would often use the majority of the route you're talking about. My length with toad is about 4' shorter than you. As long as you're not in a hurry I would highly recommend this route just for the scenery. The approach into Lewiston is a little tight, but I know you're a full timer and experienced driver so I don't think you'll have any trouble at all with it, and the view is worth the tight turns.
My biggest learning curve on the steep portions of that route was how to deal with the logging trucks. I don't have the power plant that you do, but I could easily find opportunities to pass them on their slow ascents. The problem I found is that they are very familiar with the highway and could handle the descents at a high rate of speed. Of course I wasn't descending anywhere near their speed, so consequently they would wind up with their front bumper a few feet from the back of my toad. I found there are some straight sections that you can pull slightly to the side of the highway, and they won't waste anytime getting around you.
It's a drive that should be on your bucket list to do at least once.;)