Hi,
All of the original 18th century English fowlers that I own have standing breeches and barrel keys and only one is a high end gun. The rest are middling quality at best including a gun by a provincial maker not in London. The vast majority of 18th century English fowlers I've examined in museums and collections made after about 1750 or so had standing breeches and most of those were of export or middling quality. The breech on the high end gun I own by Heylin and mounted in silver is about 1 3/16" wide and made in Spain. It is not especially massive but the key feature in all of them is they taper rapidly from the breech. Unfortunately, the only barrel that used to be available that was close to being correct was the Griffin profile in "D" weight made by Colerain, who seems to have dropped off the face of the earth. It is hard to find a modern made fowler barrel with breeches more than 1 1/8" wide.
dave