There are some pretty wide pullouts along US 12, especially in the stretch from Powell west to where the Lochsa River joins the Selway River. You should have no trouble parking in them, some are a long distance (30-50 ft+) away from the road, and traffic is pretty light so there's not much noise to keep you awake. Maybe check with the folks at the USFS Visitor Center on top of Lolo Pass, if it's open when you drive through. Yes you can make it from Missoula to Lewiston in one day, but the drive is so beautiful and interesting it would be a shame to get in a hurry and try to make it in one day. Watch for small rocks on the roadway from rocks rolling down the hill on the inside shoulder, especially if it's been raining. The highway maintenance guys do a real good job keeping the roadway clear at all times but sometimes a small rock or two rolls down after they make their road patrol sweeps. As I noted before stop at the DeVoto Memorial Cedar Grove as you head west, hope you can stay at the state park at Hells Canyon Gateway in Lewiston too, a real nice place.