Doug,
Welcome aboard... You will enjoy Jim's kit very much - I have a few questions for you. Do you have an actual rifle you want to copy? As you start to inlet your barrel and align your lock, it would pay off not only to do a little reading in "Technically" how it is done, but have many pictures of the style or an actual rifle you want to try to copy. For me, My Chambers Rifle is my first Chambers kit, and I made the choice to copy one of Jim's finished rifles. O.K. - I reached way too far, but that's just me.
Having many pictures of one particular rifle will help you on the distances, angles, depths and styles you can adjust in your kit, and it helps to "Match" your progress to the final example. Jim and Barbie will help all they can, and a few of the forum members have also built his kits.
Here is a "Progress" shot of my Chambers Rifle:
I shaped the end of the Barrel tang First, before inletting, as that drove the shape of the wood inlet area of the Tang.
Forget the carving pencil marks for now, but you can see the depth of the barrel, the Tang and the inset area and the curve I had to put into the tang to fit the curve of the stock.
So... There are some key areas that have to line up. (All the books you have will say this, and I'm no expert at all) yet the lock should align with the barrel so when you insert the touch hole liner in the barrel, it's at the top center of the pan, centered on the barrel and about 1/4 of an inch in front of the end of the Breech Plug. That's why I started with the barrel and adjusted where the tang is, using the lock as a guide. Those two pieces are kind of "Tuned" to each other, and when they are aligned, you can adjust the other pieces around that fit. I did move the barrel back a little (1/16 of an inch, and with the pre carved stock, there is a little wiggle room for setting in your lock for alignment. As others have mentioned, Take a dowel or the ram rod stock and insert it into the barrel and measure 1/4 inch in front of the face of the Breech Plug, use a sharpie and mark that location on the outside of the barrel on the Lock side. Right in the middle of the "Flat" of the barrel. That will be your target for the top middle of the lock pan -
So Get some pictures, get your hands on a few actual rifles, find your "Goal" project and get as much info from it, and ASK the questions, especially if you can connect with the builder of the rifle, I have found that builders are very willing to help.