Welcome to the .458 Lott club Tim. It is a great caliber. Yep it kicks, but not all that bad. About the same as the .470 NE in a rifle of decent weight, which, if I remember well, the Mk II RSM, is at 10 1- 1/2 lbs, and that it a good thing.
Beautiful rifle, especially the barrel with its integral quarter rib with NECG express sights, barrel band front sight and barrel band swivel mount. True CRF too, integral scope mounts, 4+1 capacity in .458 & .375 if memory serves, and OK safety (although not quite perfect), a lot to like, and I personally like the really beefy stock too. You will see, the recoil lug arrangement is a bit complicated, but it works. I have never seen or heard of a report of a split stock on a RSM...
Congratulations!