I really dont know what '45-70' Frank Barnes would have been able to use 45Colt bullets in (WAY too small unless you paper-patched up).
Also, FWIW, as good a reference as his 'Cartridges of the World' is there are some discrepancies from actual cartridges (or in editing/printing things became unclear or incomplete; it does happen). What Im saying is use it for what it is, but dont take it as the 'last word'.
RFD, do I remember correctly that W231 is good for top loads in some handgun cartridges but is not recommended for reduced loads? Ive never used it but it has been around like forever. I think I never got any because I didnt want to make top loads. Ive no idea what it might do in such a voluminous case as a 45-70, maybe bad things.......
Bullseye, RedDot, and 700X Ive used for a number of reduces velo, cast bullet loads, including RedDot in 30ish cal. rifles. They are all on that quick end of the spectrum and a little does a lot, and the risk of double charging is not to be taken lightly. They, as well as Unique, are 'shotgun' powders, so all pretty fast. You will readily find data for Unique and cast bullets in (I think) every caliber known. I somehow expect that an online search may turn up some loads for those faster powders and if so I would carefully consider the level of loads they are talking about (ie. light, mild, or heavier/faster velo) and 'if' I wanted to start low and experiment. There is always a risk when off 'book'.........the good thing about cast bullet 'soft' loads is that dangerous pressure levels are not often achieved. Too light and the bullet fails to exit the bore (bad, but not horrible) and up to, say, 1100fps (sub-sonic) or even a bit more with lead is what I would look for if I were you.
You, of course, as always, are totally on your own when venturing 'off book'.