Brandon: first welcome to the ALR! Nowhere better to be.
I rebuilt a GPR flinter for a friend, though it was not a kit but a finished rifle I started with. Nonetheless, there are solme idiosyncrasies of this rifle that lend themselves to custom work, and in my eye, a great improvement. You are working now on one of those.
The standing breech - that part of the tang that is screwed permanently to the stock, and from which the barrel lifts away, has two negative things. First, the sides of the tang are filed off creating two narrow facets that do absolutely nothing to improve the architecture of the rifle. And second, and something my observant brother alluded to, is the factory tang has a straight section aft of the break-off, before it starts to curve downward. In the picture above, I have bent the tang down gently, immediately behind the vertical part, to start the tang's decent into the wrist immediately at the breech of the barrel. Failing to do this will give you a stock profile that is unattractively thick right behind the barrel. Now, since you've bent the tang down a little, you must also straighten out the tang where it IS bent, so that it is the correct length to fill the inlet. Now, attach the tang to the breech of the barrel, so you can inlet the two as a unit. This will take you a little deeper into the wood, and at the same time give you the opportunity to file away those two little bevels they had on the stock tang.
I wish I had better pictures to illustrate these concepts...this is all I have.
Good luck, and don't hesitate to ask questions.