I suspect that Pedersoli is doing a little Lawyering to protect itself from people using their barrels to make something else, and the law suits that will occur once the firearm passes through several hands, and several decades. If you have to cut the two barrels apart, then remove the plugs, and then convert them, there will be enough evidence of after market altering for the company to have a secure defense to such a lawsuit.
I have plans to make an over/under shotgun, using modern steel barrels. These plans are for a straight line percussion action, but a flint action would also be possible, although the width of the gun would be rather wide. And, people should also consider a swivel breech action for a double barrel shotgun, in addition to the S X S configuration.
Finally, while a 12 gauge double seems to be the ticket, the smaller gauge guns are handier in the field, and will take game birds just as readily.
I had the opportunity to hold and examine a 12 gauge Tower English DB Shotgun, from the 1770s many years ago. The barrels were 36 inches long, with no choke. The buttplate was almost 3 inches wide' the LOP was about 12 1/2 "; the drop was closer to 3 inches at the comb" and the forend grip was a wide, 3 1/2 to 4 " wide , piece of wood. The gun balanced right at the forend where it felt the most comfortable to place your hand. That fact made the gun very easy to swing. The Tower locks used the military style double throated cocks, and bess style lockplate. The gun was owned by a private collector, and there was no doubt as to its authenticity, age, or the fact that it was made for an English officer who took the gun to the Colonies during the revolution. How it left his hands I did not learn. It did weigh quite a bit, but I did not have a chance to put it on a scale. The fact that it balanced so well made the weight almost unimportant. I think that will be the key to whether you like the S x S double gun you make, or acquire. I have handled many original side by side double guns at Friendship, looking for something that fit, and I could afford! So far, I have not found a gun, made in this country or abroad that balanced as well as that s x s flintlock double gun.