Gert,
Might I suggest driving a bit further on the 29th, so that the 30th isn't a grueling, un-enjoyable day. For example, we could continue past Cour d'Alene to Moscow or Lewiston. That would shift a hour and a half of driving from a really long day to one of the shortest.
Likewise, on the 1st, we could continue to Whitehall or Three Forks, which would shift about an hour from the 282 mile day to the shorter 127 mile day.
Poking at your itinerary some more, how about stopping in Red Lodge on the first day to shorten that day a bit--first day is always a delayed start, and who knows what the traffic hold ups will be in and around the park. The following day seems to be pretty flat and featureless terrain, so wouldn't be affected much by some added miles.