I'm sure there are gun builders who could build a rifle that would fit you but, the really good gunsmiths who specialize in muzzleloading rifles usually have a year or more backlog. They are also very expensive.
Since you are talking about building your own gun, I suggest that you call Pecatonica River Long Rifle Supply Co. and ask to talk to Dick Greensides. The phone number is 1-815-968-1995.
He owns Pecatonica River Long Rifle Co and has supplied pre-carved stocks for well over 25 years. He is also very knowledgeable and an all around good guy to do business with.
I mention this because when I have used his stocks I usually end up cutting over 2 1/2 inches off of the butt of the stock to get a 14" trigger pull. If I didn't cut off that wood the pull distance would be around 16" after the butt plate was installed. Anyway, maybe Dick can cut a special stock for you that leaves more wood at the butt? It's worth asking and if anyone would know, he would.
Here's a link to the Pecatonica River site. They have a lot of different stock shapes to choose from.
If you decide to buy from them you will have the cost of the kit parts and stock forming plus the stock blank cost.
Their Curly Maple has a lot of figure in it. So much so that I usually end up ordering the #3 grade.
http://www.longrifles-pr.com/stockscomponents.shtml
By the way, because of the long barrel length used on longrifles (42" is common), it isn't unusual for the length of pull to be less than what would be found on modern gun stocks. That shorter length of pull moves the center of gravity back towards the shooter so the gun doesn't feel muzzle heavy. This is especially important if you use a straight octagon barrel rather than the lighter weight swamped barrel.