@Garrett89
IF you are having a rifle BUILT to pair with the 450/400 with the consideration of plains game; I would....
Have a TAKEDOWN .375H&H bolt action made so that you could fit both rifles in the same Pelican. Let's be honest traveling with a gun can be a pain but it's really not much more trouble to carry twoguns if they don't require separate cases.
The .375 would cover all the plains game big and small and but if you wanted a second .300H&H barrel and forend for the takedown bolt gun, it's certainly possible.
Thus if you were traveling to a country that will let you bring the takedown as one gun or allow 3 guns for one person you could have 450/400,.375&.300H&H all on the same hunt and same travel case.