I make touchhole liners out of 3/8" stainless, (If barrel flat will take them)
If the right amount is threaded, they run up against a shoulder like a bought one does. For threading stainless, try dish -soap. Cuts smoother than cutting oil.
With a big liner like that, you could drill it out on an angle or otherwise move the hole and still have plenty of room to drill it out larger on the inside.
Good luck whatever you do!
R.