Justin,
It is as well this will be a 2020 project , as it is best if you read all you can, and study all specimens you can find before you plunge into this.
It is complex, and there are a million ways to get it wrong, but only one way to get it right.
Altering a commercial lock to make an inset breech is not easy to do. The pan and hammer (frizzen) need to be much deeper. By this I mean wider so that this specific area fits into the inset breech. To do it right will really mean making these parts and fitting them correctly.
See photo below for an idea on this;
It may be easiest to just file the breeches back to create a double without the inset. This can be done well and still be pleasing, especially if the breeches are patent, and the breech can safely be well filed back, see photo below.
I would credit this photo but no clue where I got it now.
I can pull a lock from one with inset breech if you like, but at this stage think it best if you go with the fit as above.
Click on photo to enlarge.