With a sound gun, and blackpowder, don't worry. Fill the case and aim true.
To allay any fears regarding the strength of the Rolling block action, I will quote a passage from Harold L. Peterson's The Treasury of the Gun (C. 1962 Golden Press) pg. 182 & 184.
"With the ammunition available it was literally impossible to blow out a Remington (Rolling Blocks) breech. Certainly the Belgians tried. In the world renowned proving house at Liege, Belgium, a .50-caliber Remington (Probably a .50-70) was loaded with seven hundred and fifty grains of black powder, forty balls and two wads so that the barrel was completely full and would hold no more -- a charge more than thirty-six inches long. It was fired and the director of proof noted laconically that "nothing extraordinary occurred."
(Parenthetical remarks added for clarification. LMH)
I do not recommend any one attempt to duplicate the proof test, and if you do, I disavow any connection to the experiment. Neither would I use modern +P and Magnum .45-70 loads in a Rolling Block, nor would I attempt to hot-rod a .45-70 nor a .50-70 or .50-110 with smokeless powder. But with today's modern metallurgy, I doubt if you will hurt a Rolling Block reproduction with blackpowder, nor place undue stress on an original gun.
BTW, I agree with FAL308 and Trapdoor Billy about not converting your No. 5 Carbine. You'd never realize as much on a converted rifle as you could make on one in original condition.