The Ruger .45-70 single shot rifles are terrific fun! I shoot the Number One and have shot several Number 3 rifles many times.
Several years ago, a Canadian won the national black powder cartridge rifle long range (600-1000 yards) competition using a stock Ruger Number 3 with adjustable sights. This was the FACTORY 22" barrel!
True, the .45-70 kicks noticeably in the carbine-style number 3, especially when using heavy 500 grain loads. The action is fully as strong as the number 1, and that can handle loads approaching .458 Winchester Magnum energy and velocity!
It should be very manageable for you using blackpowder loads.