You need a pre WW2 Winchester model 92/94 replacement stock. On the chiappa there is a nut that interferes with a bit of the wood on the inside of the tang of the replacement stock, about 10 minutes worth of work with a dremel to releave it a bit. There are 3 common configurations of stocks that you can buy, carbine (curved stock), rifle (curved stock), rifle (shotgun style straight butt). Which ever one you want make sure it is a straight stock and not a pistol grip stock and you are good to go.
I went with the carbine style to keep the LOP down a bit. I cut the stock straight like the rifle and installed a small recoil pad and then covered the butt end with a leather shell holder that holds 5 rounds. Nice short and handy , love it.