Just make shotshells out of your 45/70 cases.
All you need is the primer, some powder, use fiberboard , cardboard , felt or cork discs for a wad, your shot and seal the end with a cardboard disc held in place with wood glue.
That is how I make some loads for the .410 using .303 british cases that I reformed with a set of punches, and then thinned the case rim so the breech will lock up.
I also make .41 loads for the .410 out of .45LC cases resized to .475 in a homemade sizing die plate, and I load them with .41 caliber lead bullets for the .410 shotgun.
I know, you can buy .410 Brass Casings.
But I like to dink with things that are outside the box.
A 45/90 casing will not fit your chamber length, so it gives you no advantage trying to make a shotshell out of it.
But shotshells will shoot like crap out of a rifled barrel.