The photos will explain.
The excess headspace condition, it's actually greater than shown here because I couldn't hold the bolt all the way to the rear (against the locking lugs) and also take the photo.
The breechface as it was,
After machining for the new insert,
The blank insert installed,
After machining and filing, the amount of excess clearance is evident by the amount that the insert stands above the boltface.
The finished job,