Off the top of my head, NOT good at all. Of course, I once found a stock for an 1884 experimental rod-bayonet trapdoor, so miracles do happen.
It MIGHT be practical to have a liner put in - of course you would have to mimic the Pope system (barrel offset high at breech) since duplicating the SA extractor plate design would not be worth the expense. However, it then would not be "right", so you'd STILL have a problem. Hope you didn't pay any premium for the rifle, because, basically, you have a .30-40 - worse yet if you really want to get depressed, you do not even have a "correct" .30-40.
Please note that the above is NOT intended to be mean-spirited, simply a reinforcement of the #1 rule in acquiring "rare" guns. KNOW what you are looking at and ask questions BEFORE you buy. If I'd had the chance, I'd have advised against the purchase, unless you would be getting a significant discount for the mismatch.
I never believed there were SO many STUPID people in this country. Start working now to take the Senate in 2014.