I am fairly sure that the bloke below is the one who made the 35 Sambar and that was for Guns and Games.

http://www.kopjerifles.com/

At the site below, Australian Hunting Net, there is a poster called Con who I think is fairly well up on the WSM widlcats.

http://www.australianhunting.net/index.php

Provided you are wanting a 375 or 416 WSM with no other changes except necking up then all you need is a gunsmith wirh a 300 WSM reamer that has a removable pilot and neck/throater in 375 or 416. Redding a good bed for quality custom dies. CH are cheaper but their repuation is a bit "iffy".