This might be a little crude, but if the barrel hasn't been finished and still needs draw filed, try peening the unexposed flats first where the breech threads are with a small sledge or the flat end of a ball peen hammer. Just a couple of good hard licks on each flat against the anvil. Try to hit good and square and not hit the edges of the flats. Unscrew the breech plug and file the boogers off the flats. Hopefully, you will just have to pound on 5 of the flats. Object is to microscopically displace thread metal.
Bob