Bonanza
I have shot a giraffe with North Fork .458 Cup Points and Flat Point Solids and and 2 elephants with his Flat Point Solids in my 450 No2.
They are excellent bullets.

I think they are perfectly safe in double rifles.

I am very happy with my scoped 450/400.

I say go for it. I would have JJ Perodeau fit claw mounts to the "modern" 450 No2 bbls.
I can highly recommend the S&B 1.1-4 with the flash dot reticle.