Nick/fiatcrew,
As mentioned before, ,450 3 1/4" BPE and .450 3 1/4" NE are the same case. Be aware that back in the BP Express days the chambers were not standardized and brass sized in modern spec reloading dies possibly may not fit in an early BP Express rifle.

The .470 brass is relatively easy to find, the .450 does take some diligent search and lucky timing.
If you do not already have them, the reloading dies are another issue. Once again .470 NE are not THAT difficult to source, but the .450 3 1/4' can be scarce.

BTW: I personally know Huvius (Ben) and he is honest to deal with and a great guy. No worry buying brass from him.

Have fun and good luck.

- Mike