Hi James,
I understand that, but I built a fowler a few years ago with that same tang from TOW with a Durs Egg lock. After doing a bit more research I realized the sighting hump probably was not as appropriate on a late 18th century fowler having a late 18th century lock like the Egg. Consequently, I ground off the hump and fitted the tang very nicely to the barrel. Anyway, if the TOW kit has a round-faced mid-18th century lock then a hump is entirely correct.
Jim C. - Regardless of how you handle the hump, you want the hook to be snug in the tang. Position the tang on the breech hook so that the hook snugs it against the barrel properly, soft solder it in place, and then file the flats flush with the barrel except for the top flat if you want the sighting groove. In that case, file the excess metal down to create the forward curve of the hump such that it merges with the barrel flat nicely.
dave