Im a long time flint & perc BP front loader shooter too, and some BPCR and more.......I have mostly decided that the BP will be used in my stuff that requires it, and those suitable to smokeless will get such. The TrailBoss loads I use basically duplicate the BP ballistics and felt recoil of those cartridges I have directly compared. In the 45-70 that basically means you get an 1873 Springfield 'trapdoor' equiv. load and more economically than using BP too. Unique has a 'universal' 45-70 load of 13.0 grains (weighed, NOT bulk) that has worked well in lots of 45-70 rifles and is super economical too. There are other 'fast' smokeless powders that work for cast bullets too, and they fall into that 'shotgun powder' end of things, so if you have a shotgun reloader friend or club nearby you may be able to get some to try. Dont get me wrong, there are 'rifle' powders that work very well, these I mention will get you going and provide a lot of bang for the buck.
As to Loading BP cartridges, there is a plethora of info on this and it can get confusing. Take a look and bounce your ideas here first and we will try to help you weed out the if, ands, or buts of it all before you stuff up a bunch or head off down the wrong rabbit hole.