I agree with those who say there is no good reason to put this circa 1875 .450 BPE double rifle through the rigors of Nitro Proof.
And if you are contemplating having it reproofed to .450 Nitro Express pressures, then the answer is not jut no, but hell no! The .450 NE operates around 44,000 psi, so the proof load would generate roughly 50% over that, or 66,000 psi. The .450 BPE operates around 23,000 psi.
If it fails proof, you are left with a junk gun and the cost of reproofing. If it survives reproof at those pressures, you will be operating it at pressures and recoil levels that it was not designed to withstand.
Oh yeah, nobody mentioned that there is a good chance it won't regulate with higher velocities and heavier bullets anyway.
I started out using Nitro for Black loads of IMR 4198 with Dacron filler in my .450 BPE rifle that I believe was built in the 1930's. But after I read more and more about the risk of chamber ringing with the NFB loads, I decided to switch to Swiss Black Powder loads. I've been cleaning my flintlock rifles and black powder revolvers for years, so it is no big deal. Like failing reproof, once the chambers bulge or ring, there is no going back.
