Have hunted Alaska several times including brown bear twice. My suggestion is to ditch the waterproof boots all together and go with a pair of breathable waist waders and non-felt soled wading boots. If you want to sit down on wet ground, fine, need to bust across a creek to get somewhere, you are ready. You won't be doing any serious mountain climbing and the wading boots are comfortable.
Take 2 rain coats. 1 that is breathable for when you are walking and a Helly-Hansen Impertech Guide Coat (goes down to about your knees) for when you are sitting. You can literally have rain the entire hunt. If it is breathable, it will eventually leak/wick through but nothing gets through the Helly Hansen but you don't want to walk in it, you will sweat to death.
A 338 is plenty of gun. I used a 250 g Partition and was one and done straight down on both of my brown bears. Shots went through and through. Your guide will probably be carrying a 375 or larger if you get into a pickle and need DLP backup.
