For what its worth:
Looking at the pictures, I can't tell if the barrels elevation is in the right position relative to the lock or not. It may be an illusion that it is low because of the amount of wood at the breech and the height of the wood on the sides of the barrel channel relative to the center of the side barrel flat.
In other words, if you inlet the lock and it is positioned way above the center of the barrel flat, you do have a problem.
If the top of the pan (for a flintlock) is centered to the center of the barrel flat then all your looking at is some excess wood that will have to be removed.
If indeed the barrel is too low for the lock mortice then it will need to be raised.
One method that could be used is to buy some Maple Veneer which is available at most quality wood working stores.
This stuff is about 1/32 thick and 3/4 inch wide and comes in a roll.
I've tried the type which has a heat activated adhesive on it with limited success. If they have it without the adhesive, I would choose it and epoxy it in place.
If you do decide to Epoxy the veneer in place, I would suggest that you wrap the barrel with a layer of Renolds Plastic Wrap to prevent the Epoxy from sticking to the barrel, then place the barrel into the stock and lightly clamp it to hold the veneer in place while the epoxy hardens.
If you split the veneer so it is the same width as the barrel flats, you could put two layers on the bottom stock flat (1/16 height) and 1 piece on the 45 degree flats. That would create a .012 clearance between the 45 degree material and the 45 degree flats on the barrel.
If you don't like the idea of the clearance, you could use two pieces on each of the 45 degree flats but you would then have to remove .018 of material off of them for the wood to contact the barrel without lifting it off of the bottom flat.
Personally, I would opt for the two on the bottom and one on each 45 degree flat. No one will see these 45 degree pieces anyway unless the barrel is removed, and then they won't know how much clearance is in there anyway.
This would give the stock full support on the bottom and two side flats.
As for sitting the barrel back 1/2 inch, if it is a straight barrel that should be no problem. If it is swamped, that could cause difficulties.
To me, it doesn't look like removing material at the top of the tang area would be too big of an issue. Yes, there will be less taper or slant between the top of the barrel and the lock panals, but it looks like 1/16 could come off of them leaving about 1/8 inch of panal width.
If the lower panal width was reduced to match the upper one it would still have more width than #66, 67, #68, #70, #74, #75... in Shumways ROC Lancaster section.
Bottom line? I think you can still come up with a beautiful rifle which looks "very" correct.