Depends on where you buy the ammo, or whether or not you reload.
If you compare standard pressure 45-70 Trap-door, 30-06, and .458 loads, you are correct.
But if you have a modern single-shot or lever action, not so much.
rc
This is what I was looking at:
Buffalo Bore 45-70 Lever-gun ammo:
500 gr. FMJ bullet - (1,625fps / M.E. 2,931 ft.lbs.)
Buffalo Bore 30-06
180 gr. SPTZ bullet - (2,850fps / M.E. 3,245 ft.lbs.)
Hornady 458 Win Mag
500 gr. Solid - (2,140fps / M.E. 5,084 ft. lbs.)
Like you said, it depends on what ammo you are comparing. Regardless the 45-70 is really close to the 30-06 in terms of energy and a lot less than the 458 Win Mag. 58% by my calculation (about half).
Anyway, my point was that the 45-70 is a lot less powerful than people make it out to be.
As far as recoil goes:
Winchester Model 70 30-06 7lbs. (26 ft. lbs.)
Marlin Guide Gun 45-70 7lbs. (48 ft. lbs.)
Ruger #1 458 Win Mag 9lbs. (66 ft. lbs.)
Ruger #1 458 Lott 9lbs. (76 ft. lbs.)
Also just because it's my fun gun to take to the range:
Weatherby Mark V 460 Wby Mag 9lbs. (113 ft. lbs.)
Maybe a 45-70 out of a light weight rifle would be enough of a step up from the 7.62x54R for Ryanxia, but I still think the Ruger #1 in 458 Lott would be the way to go.