My western (and Eastern big river also) rod is a 9' 6" 5 wt Z-Axis. Starting using it when fishing with a guide on the Truckee and made one up from a blank when I got home. Have since used it all over CO and MT, and the upper Delaware.
I like the little longer length for mending long casts and having a little extra length for nymphing. Also, I think longer rods help pick up more line on a long cast to set the hook,
As far as weight, there are days when maybe a 6 wt (or heavier) would be better. I'm not sure one weight means all that much, so if I needed a heavier rod I go to a 7 or 8 wt. For example, a heavier line may help toss big foam salmon flies or grasshoppers better. Also, if you are going with those big articulated streamers a heavier line is helpful. As far as backbone, I lost a hog once on a tailwater withthe 5 wt that ran around a sharp rock. I'm not sure if a heavier rod would have helped to turn it. Have caught decent sized bull trout and Pyramid lake cutthroat on the 5 wt.
In general, don't feel intimidated - it is still trout fishing. Being higher elevation, larger rivers stay cold and the scenery can be fabulous, but finding fish and presenting a fly are still the same. Used to go to CO with a bunch of PA fishermen and their skills translated just fine.
Good luck.