In some cases the WSM brass won't clear the feed rails without alterations. One approach is to install a mag box with its own "feed rails" which are really feed lips. The reciever has to altered either way.
If you chose the SAUM or RCM cases they will fit without any feed rail alterations. The case capacity is a bit smaller but with high BC bullets you will have some great downrange performance with longer barrel life and less recoil than the huge cases.
Last year I did an experiment with the 338 RCM and had it long throated for a LA I had hanging around. I really wanted the 250 Berger VLD but they haven't made it yet. So I experimented with the 300 gr SMK and when Berger came out with their second generation 300 gr Hybrid I tried it too. Both 300 gr bullets shoot wonderfully and the OAL fits and feeds fine from the Mark X action. Velocity is low as is the powder charge but downrange wind drift is very minimal for this strange package.
57 gr of RL-17, 2575 fps (29" barrel)
I think the 7 SAUM would be a great chambering. Make a dummy and figure out where you wnat the 180 or 168 VLD or even the 162 A-max to be seated then have a reamer made to your specs. The 7 SAUM is high on my list to do next.
Let us know what you end up doing with lots of details.