I do not neck turn my dasher brass, but I do run turned necks in all of my other BR chambers.
If it were me, I would iron out the necks of the stock 6 BR case and turn those cases all the way to the neck/sholder juncture. This will eliminate any chance of a doghnut forming down the road.
After you neck turn the BR brass I would use step 2 from the above link to FF my brass. You will end up with new dasher brass that have been neck turned well into the sholder. I ALSO Jam the bullet hard into the lands and use LOTS of neck tention. Your biggest enemy is the brass being slammed forward by the fireing pin, giving you lots of run out in your FF'd brass.
Good luck,
Tod