Dave - Since this whole idea came up I have had a chance to shoot a #1 in .416 Rigby and my 45/70 back to back.
Federal factory ammo in the .416 and 500 gr.JSP/"Ruger #1 only" reloads in my 45/70.
The .416 was much easier to shoot - the extra weight in the barrel greatly reduced the muzzle jump and made the gun much faster to reload. Felt recoil seemed about the same to me although the .416 had about 6 ft. lbs. more on paper.
This plus the twist problem made rebarreling a better choice plus it would allow me to use the thin rim brass.