The blueprint which was designed for a narrower barrel has to be modified obviously. Just use the blueprint for a reference not an actual guide.
Basic design layout is nearly always the same.
Start with your barrel and the breechplug.
Sketch in the topline of the barrel inlet at halfway up the side flat. You can plane this down later as desired to show more barrel.
Figure out where the touchhole will go.
Now position the lock tentatively based on the touchhole.
Add the web between the barrel bottom and the ramrod hole, add the ramrod hole, and 1/8" below that.
Roughly position the trigger based on the lock.
Now based on the blueprint or pictures of Armstrong guns, place the buttplate for length of pull, drop, and buttstock style.
Now sketch in the comb, wrist, toeline to "look like an Armstrong"
Now sketch in cheekpiece etc to fit well in the buttstock
Go back and re-position the lock so the tail centers better on the wrist.
You should now be good to go in the buttstock section and can locate the entry thimble, other thimbles, underlugs, sideplate, sights, nosecap, etc.
etc. I think Eric Von Achwge did a tutorial on making blueprints
