I suppose a tapered bore could be just that - a tapered bore, but, are the grooves full depth and not tapered with the bore?
Meaning, do the grooves stay at .458" while the bore tapers from .450" to something like .446 or so?

If that's how they were done, I see no reason why a conventional cup and core .458" jacketed bullet wouldn't work.
The Hornady 350gr RN are supposed to be a good choice for BPE 450s.