Roundball, I don't know enough about the question to vote, and I don’t know if you will consider this info pertinent, but since you said muzzleloaders, not just rifles, I’ll pass it along. This concerns mostly fowling pieces, not rifles, and is England, not America, but would tend to show the sophistication of the gunsmith’s trade at that time and place.
Writing in 1789 in London, Wm. Cleator discusses the importance of the length of the gun overall, the length of the buttstock, or LOP, and the “bent” of the stock, all as related to the size and build of the individual shooter. He says these “...can be determined with great accuracy by the gunsmith, from observing the manner in which the shooter presents his piece and takes his aim.”
Writing 20 years earlier, 1767-70, again in London, Thomas Page goes into much greater depth in his discussion of the proper fit of the gun for “shooting flying”. Too long and convoluted to quote, but here’s a list of some factors he speaks of when advising a shooter on choosing a fowling piece:
barrel length and its relationship to quickly and accurately shouldering and aiming the piece. Shorter is quicker but more difficult to keep on target, longer is slower to mount but more steady in aiming because of increased sighting radius. His recommended shorter gun was 33”, his longer one 39”.
Overall balance, with barrel thickness and profile discussed
Length of pull, breech to butt, ranging from 14” to 15.5”
degree of bend of stock, dimensions given for average drop at heel and at comb, and description of what is needed for tall, short, heavy and slim, long- and short-necked shooters
castoff advised for wide-shouldered shooters
need for the gun to automatically line up on target when quickly mounted, description for how to test that
And once the gun is in the hands of the shooter, advice to replace the flint with a wooden dummy and spend a month mounting and dry firing at any old target to become accustomed to the gun.
Now, how much of this sophisticated understanding of gun fit was mirrored in the colonies I leave to others to figure out.
Spence