Jason, I think you hit the nail on the head stating 'STOCK'.
The exhaust that came on the Batmobile ('75 2.0L) had 1.7/1.8L heat exchangers and an aftermarket muffler system. Also, didn't receive front or rear valances with the car, acquired them a couple of years afterward.
When we attempted to install the rear valance, we found the tailpipe exited too far to the right of the cutout (closer to the license plate). The two other exhausts I acquired were also aftermarket and exited too far to the left
We're going to install 73~74 2.0L SS heat exchangers and a dual-outlet Triad for now (well, as soon as I get my hands on a correct 73~74 2.0L muffler hanger) and we'll run without a valance.
I was fortunate to pick up a pretty nice stock 2.0L banana muffler from Glenn at the G&R ... I'm hoping that will work with a stock valance.