Decreasing COL will increase pressure, but FWIW, Quickload software predicts that load to be 32162psi based on the default case capacity of 79gr water, which is well within the rifle's capability, H&R recommends 1895 levergun loads for the SB2 framed 45-70s, Hornady and Hodgdon both set levergun limit at 40kcup(40kpsi), so you'd easily be good to go. Be aware that H&R chambers the 500S&W in their rifles also, same frame as the BC, it has a SAAMI MAP of 60kpsi, and most data goes to about 53kpsi, so even using modern rifle 50k 45-70 loads in the BC isn't going to be dangerous, although it will be dangerous to the guy shouldering it.
My best load in one of my BCs is H322 and the 350gr Hornady
RN, I throat reamed the barrel so I could load to 2.555" tho.
Tim