Agree with the others on most points. I like the PT&D Obendorf bottom metal and a factory mag box should be fine, a good smith can make it feed flawlessly. Order early as PT&D works on a rotation basis and if not in stock could be months for delivery.
The shank gear it to the stock as BD and other posted or even shorter for weight reduction. A Bartlien #3 seems portly to me a 2 or 2B seems more reasonable especially for an elk rifle, but this is personal preference and any barrel can be balanced out with a little weight. Another Trigger Tech fan here too, they have a good reputation for working even when gunked up. Way better than a Jewell which I have personally knocked out of commission.
You may want a barrel band swivel especially if you don't shoot with a sling. Keeps the barrel lower when shouldered, sits on the bags more easily and you will never nick your hand if you reach forward excessively.
Recoil lug as posted is more for the tolerances but while you are at it might as well go 0.25" for added stiffness.
I would not get Mag Fill although I believe that the 338 is the cut off point for McMillan recommending the extra fill or actually I think it is extra glass. Might ask them if they have seen any issues with this, but they will advise on the cautious side, but it could affect any warranty work.
