dave
I have had a No1 in 45/70 for about 30 years. For the last several years I have been using a 9,3x74R Double rifle.
I feel the 9,3 is everybit as effective as the 375 H&H IMHO.
I have taken a lot of game with my 9,3, including giraffe cape buff and elephant.

The 9,3x74R would be an excellent calibre for the No1.