If you do some research you will find that the 1885 Single Shot is one of the more nearly indestructible actions out there.
There is a warning though. The factory, when they got one returned to them for rebuild or custom work, would re-do the heat treating on all the Case Hardened ones. And I would say to temper ones enthusiasm a bit if you were looking at short fat modern magnum calibers. Rimless extraction on a high wall is a bit of a project, and you want to have a safe margin around the chamber.
Winchester used a bunch of them in their ballistics lab well up into modern times, as test hack actions. I'd go out on a limb and say that you could use pretty much any rimmed cartridge you can find, and like as not, load it up to levels that would cause damage if shot in other contemporary rifles.
I can really suggest reading the two books by Campbell on the Winchester Single Shot Rifle. See if you can order a copy of each in through your public library's inter-library loan program. Kinda pricey to just buy if you are not otherwise enamored with them, eh.
Why not spring for a barrel in .45, that more suits the action, longer and a bit heavier? 40-65 is readily available and popular among the BPCR silhouette crowd, as is 38-55. Either would work very well for hunting, and are less recoil than the long range loads for a 45-70.
I like them.

One of these days I'll own one or two. Opportunity and timing have so far managed to dodge me.
Cheers
Trev