I can share with you on my experience with the 45-70 in a double rifle. Mine shoots pretty good with
350 t0 425 gr.cast bullets. My regulation velocity with the 425 gr. is about 1770 fps at 12 ft. from the muzzle.This bulet is around 15 bhn + or - . This load penetrated a large american bison directly through the sholders and kept on going. The wound channel was large with around a 2" exit wound.It is a lot of muscle to get through and keep on going.It is not a long range round, but if you do your part it is well up to the task in North America.
I am sure that armchair ballisticians would have me hung for the last statement but results speak much more athoritively than words-seeing is believing, my guide was pretty suitably impressed as well. As for longer length 45 cartridges the will work well also. You can get the same results with noticably less pressure or run it up and get increased performance if you and the rifle can stand it. Going up from 45-70 perforance I run out of durability quickly. Let recoil decide what you can stand-not ego. Best of luck on your project.