depends on what equipment u have to work with and how carefull u are , cutting the bottom of the sleeve shoulder is the easiest, but every time u change cyls u have to match the new to the old. the cyl is problt tapered so u should use an expanding mandrel
to cut the Case i use a 4 jaw chuck and a piece of aluminum big enough the mount the engine on the end like u were mounting it in the plane, put the setup in the 4 jaw chuck and indicate it with an indicator now the important part, remove the case and take a skim cut on the fixture ,remount Case and finish indicating to zero run out use a small sharp tool and remove the amount u want to take off .when done put a small chanfer on the edge so it wont bind the case.
the more back pressure u have ,the more it affects the sub piston induction, when we were kids a friend of mine cut the wrong side of a racing engine and u could see the rod under the piston and it ran fine hope this helps
dont forget that u need to raise the head the same amout that u dropped the sleeve