I always file the draft on the tang and lug of the breech plug at the same time I'm filing the finial on its end. I install it into the barrel, and inlet the plug with the barrel attached. For me, it ensures that I inlet it straight in line with the barrel. If the tang is tapered, ie: narrower at the barrel than at the other ed, remember that as you go back toward the butt, you will leave space alongside the tang where there was once steel, so go straight down if you can. That's where you'll enjoy working with that draft you took the time to file in.
If you have an inlet that offends you, simply glue in a block of matching wood, or a couple pieces along the sides, and have at it again, after you have filed the draft. Only a few degrees of draft will make the job much better and far more enjoyable. Once you dress off the tang and wood together after inletting, stain and finish, you likely won't even see the joint(s).