jeff, i have sleeved plenty of masts before, always an internal sleeve 600mm to 1000mm. using about a dozen 3/16 inch stainless rivets was enough. using some epoxy glue to take up any voids. never had to use external sleeve or to use cloth on it. i have heard of carbon going straight on to alloy and the alloy is completely eaten out in about 6 months. but make sure you cut the existing bits with a drop saw or similar so that the both ends are perfect.
cheers.