Why do you savages feel compelled to pull a barrel for just threading it????

Mandel to fit into the receiver on the chuck side, live center on the other end, and work between the centers for threading.
Note, for first shorting the barrel, steady rest on where you are going to thread will work fine, and not mar any of the barrel finish that is to remain.

Once you have the barrel threaded, make a over size OD donor sleeve to thread on the new fresh threads, take a pass on it while the muzzle is still in the live center to true the OD of the thread on sleeve to the bore, then use a steady rest on the sleeve to re-crown the barrel.
Note, once you re-crown, live end does not go back into the  bore crown!!!!

Done this way, you do not harm the bluing in any way, nor have to screw around with head spacing.

Plus with the threaded sleeve, and Mandel handy, going back to re crown is a snap, and don't have to over chamfer the crown for ease of back brush stroking in the first place.

Center Mandel inside the receiver, and you don't have to worry about marring the finish on the outside of the receiver by chucking on the outside of the receiver in the lathe chuck.