Makers kit "A" is the one you need. I think reaming your current barrel would be the least expensive route and you have the correct bolt face with your 338LM so nothing else to do. Tubb has the Manson Reamer for $220.00.
I really liked the idea of interchangeable calibers but truth be told, once I have a favorite caliber, the other ones rarely see the light of day. I keep my 338NM Barrel around because I love that round and it mag feeds, but the 33XC is just better for the mile plus shooting I'm enjoying. Recoil is a breeze with the Barrett/33XC combo. In fact, when I trained with Chase, he unloaded the bipod and had a relaxed hold with the buttpad just resting on his shoulder. I said, "So you take the full brundt of the recoil" and he said "yes". Allows the rifle to track much better if you don't use ski's. Now if you've seen Chase, his biceps are as big as my thighs, so I was a little hesitant, but I shot fantastically with that method and even after 150 rounds of 33XC that day, my shoulder felt like it did when it started. The MRAD is absolutely perfect for the XC. I believe I've stated before, Chase said I could take my Barrett with that Bartlein barrel and enter KO2M and place. I told him the only problem with that is I would need him spotting since I still lack the experience to manage the wind at those distances.
One more thing, you could actually use the Maker's kit for the 37XC or 41XC when they are back in stock if your lust for ELR isn't cured with the 33XC.