take out a tiny amount of metal before the 308 headspace gauge goes in all the way.
Or for $80 rent a pull through reamer from reamerrentals.com and it headspace it's self.I converted a 300Sav Arisaka to 308.
Just pull the barrel, put the barrel in the lathe dial it in, put the 308 reamer in it, and take out a tiny amount of metal before the 308 headspace gauge goes in all the way. Put the barrel back in the receiver.