The gun you are thinking of using for your conversion is rather weak for a double rifle and will in all probability shoot loose or maby not even pass a re-proof which must be done for your safety. What you want is a gun with double underlugs with double bites and some sort of third fastner. From the proof marks on a gun and with bit of math you can determine if the cartridge you are thinking of using falls within the safe pressure range of the action. Ellis Brown's book will help you out here. If you want some pointers give me a call or contact me through my web site which is customguns.us. However I would not recomend building a double rifle based on that gun!!! One other thing sleeving barrels is sort of a half assed way to do things and makes for a very heavy and unhandy rifle.