As a strict non-expert my suggestion is to post LOTS OF PICS so we can assess your best option.

I am about to start shooting a 500/450 No 1 BP Express. I feel it better to keep the original barrel and calibre if it is overall in good condition, UNLESS your possible project rifle will be something indisputably better. That means to me, a staggeringly nice stock, show-class gunsmithing in another rare calibre (British and flanged), best blacking and maybe cased up with the requisites.

The 450 BPE case is satisfyingly impressive in size. The ballistics are gentle, comparable with mild modern loads for light bullets in a 45-70. What could be sweeter to shoot? And in that action you could load for heavier bullets without the problems of regulating in a double.

See how it shoots before changing it. You may find you like, or love, it as-is. If not, let us know...