I would like to add my two cents worth here. Rifle/shotgun combinations seem like a very handy setup in theory, but in practice they don't work out so well. The problem is in where your cheek fits on the gun must remain constant and the stock drop will be another constant. When you add the top rib and the sights to the rifle barrels, that means you will have to raise your head up to meet the new higher line of sight and your shooting of the rifle will be somewhat impared. It may not be too bad with the .410 frame but you still need to try and keep the new sights as low on the barrels as possible.
Since this sounds like your first conversion, I would look for a shotgun that has good lockup first of all, then convert it to the .22 Hornet by using the barrels to make the monoblock and forget about having a second set of .410 barrels for the gun. The .410 gauge is the least useable shotgun gauge out there anyway. I guess what I am saying is that I think that on this first conversion, I would use it as a learning experience without adding in all the extra difficulty of trying to make up a two barrel set. Good luck with your project. Bob