I'd suggest that you buy a copy of "Building Double Rifles On Shotgun Actions", by W. Ellis Brown. It won't answer all of your questions but it will point you in the right direction. IMHO sleeving the rifle barrels inside of the existing shotgun barrels is "Mickey Mouse". The better way is to cut off the shotgun barrels at about 3 inches and then either solder or screw the rifle barrels into the monobloc thereby created.