Nothing wrong with the 45-70
Brass is cheap.
The 45-70 doesn't take as much powder as the bigger cartridges need to do it's trick.

The spartan comes *already chambered* in that round, shooting it as such is avalialbe to you *at no extra charge* as shipped. No fooling around needed. No possibility of screwing the gun up by rechambering it either. Remember, if it came from the factory chambered in a tidy little round like the 45-70, they probabally weren't thinking of setting it up to take three and a half inch brass...
Also consider that brass for those classic rounds usually costs about five bucks apiece.
That's one empty piece of brass for five US dollars.

You can get a bunch of 45-70 brass for five bucks.

That and once you're through rechambering a cheap double, you have what was a cheap double that might end up being a much cheaper but sellable used gun some time later in the future, but much more likely (once hacked) will be a nearly impossible to sell, very ackward piece of iron that you've sunk dough into and find that you can't get back out of.

--Tinker