Phil
I have a 450/400 3 1/4" double rifle. Two of my hunting buddies have I Hollis 450/400 3". They are very nice rifles.
If your double is Nitro Proofed it will be marked on the bbl flats. If you do not have access to a meaning of the British proof marks, an easy way to tell is the following. The bbl flats will be marked with the following: "400 express... 400 Gr MAX.....60gr CORDITE.
If your rifle was regulated for the Tropical load it will say 55gr CORDITE. Many 3 " Jeffery guns are so marked, but I have not seen a 3 1/4" rifle so marked.
Either of the 450/400's just may be the best calibre for a persons first double. If you get the rifle let me know as I have some good loads including some with 300grain bullets which work great on deer and wild pig sized game.
Before you load for any 450/400 you MUST find out if it takes .408 diameter or .411 diameter bullets.