reference materials on British fowling guns are all out of print and very expensive. I'd use Dave's tutorial as a guide. Plain English fowling guns have the same architectural base as the full dressers so his information will be excellent for any grade of gun you may want to build. For a good bird gun fit is important.
As an alternative, you may want to build a later 1/2 stock English gun. I made one for myself many years ago and hunted birds with it. It had a 12 bore 37" barrel. It handled like a modern shotgun. I have also owned several original 1/2 stock guns and have hunted with them. Again they handle like a modern gun. But, I like the look of a full stock gun much better.
I hope to build a bird gun in the next year for myself, I'm planning on a full stock gun with a 40" barrel in 14 bore (already have the barrel) with a little jug as I don't have a dog to hunt over anymore. If you want a dedicated wing shooting gun with a full stock I'd go with a barrel 36" to 46" and 16 to 10 bore with a jug. Look for a "griffin" style barrel, large breeched fast taper and light. English walnut would be nice, but a good piece of American black walnut would be fine too. Don't make it out of maple, it's an eye sore. You don't have to have a standing breech and keys....nice, but not needed. Go with Chambers early Ketland lock, probably the fastest large lock on the market.
Once you get it built go shoot a bunch of trap and skeet with it. Once you can hit most of your clays you're ready to go hunting.
If you have any more questions let them rip.