Of all the Montana's I've had, none required the magazine box to be altered. None.
8400's don't need the front base screw to be altered, only the 84L/M's "require" it. I've only done it on a couple out of the many 84's I've had, yet all of them would shoot something sub moa. Still, it's a good idea as a matter of course.
IMO, bedding the full receiver and shank (or at least the forward receiver/shank), and tuning the trigger to a light (sub 3lb) pull are the very best "fixes" for the Kimber.
Patient persistence at the loading bench, and good bench technique are equally, if not more, important.
“Perfection is Achieved Not When There Is Nothing More to Add, But When There Is Nothing Left to Take Away” Antoine de Saint-Exupery
