Mind you I have no WBY experience but I have made .338 Buster brass (It is a .375 H&H necked down to .338 with a blown out ackley shoulder. A custom wildcat my buddy has from days before the .338 Lapua, RUM and the Edge.) from .375 H&H brass. In my experience you need a case neck trimmer as the brass will thicken a lot when necked down that far and it will shave the copper off the bullets when seating really bad. I just turned the case necks until the the bullet would seat easily.
Good luck and as expensive as that stuff is, I understand why you want to go through the trouble.
Todd