When I installed Dougs on my '73 Dart, I also had a Summit X-pipe kit to install behind it. But before mocking this all up and welding it into a one-piece X, I pulled the collectors of both headers together with a ratchet strap about an inch, mocked up the X-pipe and welded everything.
But this, even with this extra clearance, I had to reposition the engine about 5-6 times on its mounts before I got just enough clearance to where I thought it wouldn't touch.
I even used one-size thinner bolts for the spoolmounts so I got some more room to play.
Of course after a week of driving around, one of the headers started rattling on a torsion bar....