Blacksmith,
I too love L.C. Smiths, BUT, they may not be a good choice for a conversion to a rifle unless you stay way down the line on pressures. The Smith has only the rotating screw that goes through the rib extension for lockup, and even though I have heard of one or two being converted to 45-70's, I would not do so myself. Many people think that they can do this if they stay with low powered loads, failing to realize that someone else could, at a later date, put a much stiffer load in it and cause a blowup.
You could get away with something smaller like a 32-20 or maybe even a 32-40 or 38-40 but then you have the problem of barrel weight.
You didn't say if yours was a 12 gauge but if it is, you are going to have to rapidly thin down the barrels as they come forward off the breech area, otherwise you'll have a gun that looks and swings like a fencepost. I have a 12 gauge Smith that I am going to convert to a muzzleloading rifle using 209 shotgun primer breechplugs screwed into the barrel stub ends and set up to shoot longer 45 caliber muzzleloading bullets. This releives most of the back pressure due to using cartridges. Because it is a pretty much contained system, you don't have to worry as much about a blowout. You will have to be very careful how you seat in the breechplugs though, you will want them just below flush so that there will be just room for the primer skirt between the breech face and the barrels. A new extractor will have to be made up that will just hook under the primer skirts. Whatever you decide to build, be sure to stamp the loading info on it in a very visable place. Good Luck anyway. Bob H.