Check piston ring end gap, just to be sure it's OK before trying to install them on the pistons.
It helps tremendously to have a piston seat tool like is shown in use on Page 8 of the 50-175 Service Manual (not shown in the 350 manual). It keeps the piston square to the bore and keeps it from pushing down into the crankcase as you try to install the cylinder over it. You can use just about any flat stock for this purpose, you may need a little tape or a third hand to keep it in place. Oil the rings and the ID of the cylinder at the bottom to help things slide together. You will need to gently work each ring into the piston groove with your fingers as you try to set the cylinder down over them, being careful not to overlap the end of the ring over the locating pin in the groove. A little motion of the cylinder, rocking and rotation, as much as is possible on the studs, will help. Whatever you do, don't get frustrated and start tapping on the cylinder. That doesn't end well.