Yes, .45-70 can be downloaded. You can probably find some cowboy action loads that would not be too harsh on the hand. I'll look in sOme of my books for cowboy load data. Is the top barrel .45-70 only?
I'd look into using TrailBoss for reduced loads. It fills the case better. I'd think something along the lines of a 45LC/45 ACP charge would work. May have to put some Cream 'o' Wheat in there to keep the powder next to the primer.
Check out www.handloads.org