I spoke with Rob West, N-SSA National Artillery Officer, who was very helpful in clarifying construction requirements. It seems those rules were written mostly in the 1950's when detailed original schematics were not widely available. As Double D has said, cannons being made then were more often made with inferior methods and material, such as cast cast iron. A new tube built to original powder chamber and breech thickness is generally approvable. So the one caliber rule does not necessarily apply if the tube is made to original standards.
I will send him a copy of this in case I misinterpreted anything.
