I made a trip out there, last summer, with BillH and our wives.
Stick to the west end of Montana. Miles City is in the middle somewhere, and boy was it ever hot and dry there. I found the best dang rootbeer in Miles City though, wish I could get more.
If you can stomach the traffic, I would suggest that you go through Yellowstone. Watch out for the idiots who park their cars in the middle of the road to chase after buffallo. I couldn't believe the number of cars that were abandoned with the doors all hanging open. If you go out the west entrance, Route 287 is a real nice drive, through Ennis, Montana. You could do a side trip there, into Virginia City.
We only had time for a few hours in Glacier. We were sad that we didn't allow more time for that part of the trip. Glacier appears to be a one stop place. Go there, stop and enjoy until you're ready to go home.
Doc