Stormson I have those same guns. Now you need to get the revolving carbine (uberti makes that one).
I never understood why they didnt put a stock notch on at least some of their 8" though? They have a stock that will fit the 12", and that would be cool too, but why not the eight? Almost every standard 8" Colt '60 has a stock notch...
I had a Pietta 1858 with 12" barrel and a shoulder stock. One of the buffalo brass frame ones. It sure was pretty with a shoulder stock on it, but I already had the carbine and didn't really need it so sold it. It would be pretty easy to add the notch on any 1858. Besides the notch, you also need the extended hammer screw to attach a shoulder stock.
On the long barrels and carbines how much velocity do you pick up?