To answer your question, I think that if you remove the barrel, use the same action, barrel, and recoil lug, and you install it to the same torque, the headspace should be the same.
To be sure - you could scribe a small line that extends from the barrel, across the lug, onto the receiver that is below the line where the stock is.
This will ensure that you have the same headspace you started with, but it won't ensure that the headspce is correct.
Now - all that being said - you shouldn't need to remove the barrel from the action just to thread it. You need a lathe with a spindle bore large enough to have the barreled action go through from the back side.
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