Having lived, fished and guided in Montana for many years may I sugest that you go with a 6wt as first choice. As many others have said that it get real windy in Montana durring the summer months due to the cool nights and rapid warming durring the day. If your going to fish big rivers on a some what windy day and fish to a feeding fish it's important to take your time to get as close as possible without spooking and make as few false casts as possible, one or two. With a 6wt line and the right leader you will be able to lay down an acurate cast into the wind much easier than with a 5 wt or 4wt. A 5 is ok but the 6 seems just right out in Montana. Also the 6 will handle weighted nymphs much better and you will be fishing them far more than dries. I perfered a soft tip rod because of the days fishing 6&7x tippett and 18 to 20 dries. The stiffer rods just don't cast small fly's and tippett as it takes a lot more line out to load the rod. Many rod makers make a medium to soft rod but they run lots of money, Thomas & Thomas and Winston are my faveroites. The softer rods don't cast weighted nymphs well but when nymph fishing you will be mostly making short casts up river and dead drifting down the seams.
Good luck have fun, the big Mo is better than ever from what I hear.