Is this an early season hunt in september or later on? Early season you may encounter snow, but it usually melts within a day or 2, so a good comfortable hiking boot that can handle a few inches of snow would be good. No matter the boot you buy, be sure and break them in by wearing them for awhile. Blisters will ruin a hunt fast!
I've always said a good bullet put in the right spot will kill anything. I've killed over a dozen elk with a 110 grain accubond launched from a very accurate Tikka 25-06. Killed a 310" bull with one shot at 435 yards with that gun a few years back so shoot often and from all positions when practicing to become intimately familiar with your gun. The best shots I've seen act is if their gun is just an extension of their arm. They are that good. Your outfitter is probably concerned about the threat of a grizzly attack where big heavy bullets and lots of energy is best. I killed a mountain goat in that area, but that was 15 years ago and I haven't been there since. If it makes any difference, I spent 6 days there and never saw a grizzly but that was then.
Most of all tho, enjoy the hunt and have an experience doing so.
Good huntin' - OH