Marrakai, you are correct in the off set bushing, to avoid makeing new fireing pins for the rimfire .22. It only requires a little extra threading or sweating in, and one must be very careful to position both barrels in the exact possition on the index. Not a big deal, but the barrels should be at the top center of the Mono-block, to make fileing in the mono-block to flow into the diameter of the barrels, and not look odd. The width wouldn't be any narrower, though!

The little centerfire .22 Hornet, would not only let you reload for the rifle, but do away with any fireing pin mods, whether in the sleeving system, or in modifying the pins themselves. With the 410 shotgun, the barrels could be left the same diameter as the shot tubes, so existing ribs could be used with very little fitting. To ballance the rifle you simply make the barrels a little shorter, and that wouldn't take much shortening when you consider the weight of 4" of both barrels, and ribs, top and bottom.

However, if one simply must have a double in .22 lr, then simply make a set of barrels from scratch, that will meet the existing pins. This would allow you to have a two barrel set, with the 410 barrels, and the .22 Lr rifle barrels. With no chamber pressure,or regulation to speak of the fitting the rifle barrels will not be very complex, for anyone with a little machine savy.