DO NOT ATTEMPT TO USE .45 LC AMMO IN A .45-70! The base diameter of the .45 LC is significantly smaller than that of the .45-70 case! Modern factory loads in .45-70 ARE set to SAAMI standards for the Trapdoor Springfields, Sharps rifles, etc. Reduced loads can be assembled BUT...the large capacity of even modern solid-head brass requires careful development for reduced loads. Faster-burning powders might seem to be the way to go, but they take up very little room in the case, leaving the possibility of the powder shifting around in the case as it is handled, which, at best won't help accuracy. I would look at published load data for powders such as 5744 and Trail Boss. Even these will have lower charge limitations, and DO NOT GO LIGHTER! Fillers can sometimes be used, but in a single shot rifle, you are probably better off to tip the cartridges nose up prior to loading. The powder will probably shift some when you insert the round in the chamber, but as long as you are reasonably consistant, so should your velocities from shot to shot.
Another thing to insure proper ignition of the powder is to FIRMLY CRIMP the case mouth into the crimp groove, or whereever you seat your bullet. The myth that a crimp hurts accuracy is just that! A firm, but not excessive crimp will help retard bullet motion until the pressure has built up enough to insure ignition of especially smokeless powders.
NEVER attempt to reduce black powder charges unless you use a filler wad to completely take up the powder space between the base of the bullet and the charge! The Army did exactly that to reduce the 70 grain charge to 55 grains in the carbine loading, rather than shorten the case. Of course with very reduced gallery or SASS loads, this might become problematic. I don't know!
Ride careful, sir!