Your 5 weight rig should be just perfect for Montana. Take along an assortment of streamers, drys and nymphs in the usual patterns. I'm sure you'll pick up a few of the local specialties when you get there.
The only real change I've made to my equipment in the last few years is sunglasses with "readers" (so it doesn't take me 5 min to tie a single knot) and wearing a lanyard instead of a vest. Really lightens the load a bit and you still have your essentials (just wear a fishing shirt with pockets large enough for fly boxes, I like the Simms "Big Sky").
This woman makes the best lanyards...
Lanyards / Accessories - Western Visions Fly Fishing Lanyards
These guys have good deals on fishing shirts...
http://www.sierratradingpost.com/mens-fishing-shirts~d~335/