I'd do as you thought, D&T, but use a bottoming tap near the bore. Sometimes, despite best efforts, you end up tapping into the breech plug .... which is OK if not for more than perhaps 0.10" .... and then you will need to use a tapered tap.
Chamfer the hole on the barrel outside before installing the liner. I use a hand-held 82 degree chamfer tool, and cut-n-try to assure the liner is a clean fit. Small gaps around the liner will likely disappear when you file the liner flush.
I like the "white lightning" style rather than the screw-in version as it can be twisted tight with a tool, then remove the sacrificial end piece.