Hi Peter,
Often it is hard to assess the quality of inletting on 18th century originals because of wood shrinkage over the years. However, I have had the privilege of examining quite a few American and European guns from that period inside and out. The quality of workmanship varies tremendously but is generally very good and tight where it shows on the outside (lockplate inlets, escutcheons, butt plates, trigger guards, barrel channels). Underneath, you often can see where they cut corners and hogged out wood. In your situation, I would bed the barrel channel with AcraGlas to fill the gap. If you build a fowler correct historically, the barrel channel will be egg shell thin on the sides. As Kit Ravenshear said, when looking down on the forestock part of the barrel, you should see almost no stock wood on the sides. The epoxy bedding will fill your gaps and strengthen the forestock so you can thin the wood properly.
dave