The barrel is .030 higher from the center of the crane in an 8 shot gun than in a 6 shot gun. The chambers in a 6 shot cylinder will be offset .030 lower to the barrel in an 8 shot frame, or the chambers in an 8 shot cylinder will be offset .030 higher to the barrel in a 6 shot frame.
DO NOT mix 6 and 8 shot cylinders and frames. It would be a cheap way to get an 8 shot gun, but that would not end well.