If the cylinder is a .357 and the barrel is cut to use that, the .38 cylinder will be far too short. A more practical swap would be a Model 19 barrel and sight if the gun wasn't machined for a Bo-
mar sight or something like that.
Changing it to a .38 cylinder would involve changing the barrel, too, because the forcing cone area of the .38 barrel is longer to meet up with the shorter cylinder.