Bill,
Like Eric says, notes from John Bivins in 1986 and details in new Moravian Gunmaking II book show the breech of the original barrel a little heavier, much of the taper about the same, the waist just slightly heavier and near same at muzzle. Getz probably made the first "Marshall" barrel for kits, then the other makers followed those specs in order to sell barrels to fit precarved stocks.
No two of the old rifles are the same, even using the same stock pattern and mounts - and they certainly were not machining barrels in those days to match each other or a "kit". Any barrel that finished up clean was put to use. I think you can build a fine copy - or a similar rifle with the Colerain barrel, or any other close to it. Not to take work away from custom makers.
I don't think it made sense then or now to try to copy a particular rifle to that level of detail. The barrel, lock and buttplate pretty well determine the size and feel - each a little different. Enjoy the building and focus on the feel and look. Have fun and show us your work. Bob