You may find that the fired cases will need quite a bit of sizing in order to hold a .458" bullet tightly.
My Westley in 500/450 No.1 Express and my Hughes No.2 both are big necked chambers but shoot .458" bullets so well that I have to give a little pinch at the mouth of the case to hold the bullet firmly, especially cast GG bullets. Your jacketed bullets should hold better though.
What I do to give a bit of a crimp at the mouth is to run the cartridge up into a 30'06 sizing die just enough to give it a little squeeze. Seems to work pretty well although I cannot back the die out enough to have a set maximum so I do it by feel and my best effort to get it consistant on each cartridge.