I was thinking from the OP that with the stock tank, you'd have to run tailpipes outboard of the rear frame rails, crossing over in an area where they'd clear the springs.
I believe Cab did that on a Duster; smaller car, but similar architecture.
But since we now know there's no OEM tank, just run the pipes inboard of the rear rails and secure them by brackets coming off the rails.
A bit narrower than OEM spacing, but solves the problem.