First try wire wheeling the threads on the plug. It may remove just enough of the carburized surface to make installation easy. Protect the surrounding surface with a few layers of tape. You'll probably get some sparks off of the steel doing this too.
If that doesn't do it,,
I'd put a tiny amt of fine lapping compound on the threads and work to plug back into position.
Sometimes things move a bit when casehardened,,,they warp.
The people that do the work have got it pretty well figured out how to avoid the worst of it and in most cases all of it.
Sometimes in the case of the threads, the very tips of the V of the thread can turn or move during the process changing the pitch (?) ever so slightly and tightening up the draw.
It takes only a couple .000" to make a difference.
You want the breech seated tight and square. Over tight is never good as it just puts strain on the threads both in the bbl and on the plug.
I worked in a restoration shop that did color case hardening and though the word was 'absolutely no warpage',,that was marketing.
Reality was refitting on a good number of occasions.
