Outlaw,
Do you know exactly how deep the breech hole is, from lip to base? And do you know exactly how much actual (complete) threading you have on your breech plug? It is common for machinists who make plugs to cut a notch ("thread relief") around the plug at the back of the threads, and sometimes at the front of the threads, as well. And some do not cut the threads in the hole all the way to the bottom, either. [I am NOT criticizing anyone here. I'm JUST making an observation.] Anyway, the point is in some plug designs and installations there may be only 1/2" or less of actual holding threads in a hole that is 5/8" or even 7/8" deep. That puts the face of the plug forward further than is necessary in relation to the actual holding threads of the plug. It's possible to rework/rebuild the plug, or install a different plug, but it's a little tedious to do so. AND, in your situation, this change would still involve moving the barrel back, and might leave gaps around a swamped barrel. But the gaps may be minimal, if even visible at all. MY POINT in bringing all this up is simply to say that working on the plug might help you move the barrel back without messing up the fit and profile of the tang area. Any time you move a barrel forward or back, up or down, you not only affect how the taper and flare will fit, but also how everything fits in the tang area as well.
As you may know already, one reason this issue comes up in relation to precarve inlets is that some of the precarves are based on original antique stock designs, which had a very minimal plug (with many at 1/2" or so, and sometimes less), and which often did not have any sort of liner. So those old stock designs had the touch hole further back than most contemporary layouts really plan for. So it is important to do careful measurements and draw everything out on paper before you start cutting. Is sounds like you did that. I just mention it here for others who may follow suit!
Have fun.
