I am finishing one right now. An unused .58 1 x 36" Green River Barrel, Manton and Ashmore flintlock. Neill Fields, who worked at the Green River Rifle Works, advised me to use a flint tang rather than a hooked breech and tang. (Track of Wolf part Plug FHG-16-3 instead of Plug-FH-16-3) . The advantage is that a flash hole liner can be installed directly into the side of the barrel ahead of the plug thus reducing weight (of the hooked breech), shortening the barrel about 7/8", increasing the useable bore length and simplifying cleaning.
Your threaded breech plug hole on the Green Mtn barrel is 3/4" if I remember right. I have cut half a dozen of them off (with a hacksaw) to fit plugs. The hooked breech has a half inch journal so you would need to also. I wanted to leave the GRRW .58 1-60 stamp on the back of my barrel, but it also had a 3/4" deep hole. So I soldered a piece of steel on the flint plug and fitted it to bottom. Not all GRRW barrels were stamped, and I could index to any flat.
In inletting the tang, I soldered it to the plug and left it that way. When I drilled for the 3/8" White Lightning liner yesterday, I pulled the plug and the liner hole cut into the face of the breech plug. I had to do this to position the lock bolt to fasten the lock. Then it was a matter of cutting the liner to length and cutting a pie-shaped notch from the center of the plug face into the liner hole. This is a lot of fitting, but all because I didn't cut that durned barrel to a 1/2" plug. In a class I took in "Antique Custom Riflesmithing" taught by Doug Roberts, shop foreman, at the GRRW in 1978, he taught us to notch the plug face back to the powder drum on a Leman rifle I built. It seems now this might have been necessary because the durned plug was too long.
The lock bolt would have to be removed to remove the barrel, but I don't plan to do that so fitted it with pins instead of keys, as Neill did on one he built. Other than the plug, there was no special building problems.