Installing headers -- While engine pulled or after re-installed?

I hope this isn't too much of a digression, but my 67 Barracuda was bought new from Economy Chrysler Plymouth in Cornelia, Georgia (same county as Clarkesville). I'm guessing that dealer is long gone.
It's not around anymore, but we moved up here from Flowery Branch about three years ago, so I'm not familiar with much of the history. I did ask about it in a Farcebook group, so we'll see what they have to say.