That's some mighty fine shooting, I'm happy when I break into the 90's.
Are you sure the Ruger Old Army uses the same barrel as a .45-70? Mine slugs at .452, essentially the same barrel Ruger uses for its .45 Colt revolvers.
Most .45-70 barrels have a .458 groove diameter. I can push a .457 roundball halfway down my .45-70 barrel and look through it and see daylight in each groove past the ball.
I've thought about experimenting with patched roundball loads in a .45-70 for informal plinking, since you can't buy the .460 roundballs that would be ideal for a .45-70 roundball load, I'm thinking of maybe .445 or .451 balls patched with .020 cloth seated in the case, wads or cornmeal taking up the extra space between the light powder charge and the ball.