Not sure on an E-body, but I fit 24" cases under an early B; that car used 3" in and 2.5" out (tailpipes); tight fit on the tailpipes on that car around springs & frame rails & gas tank, which shouldn't be much of an issue on an E-body, I think.
When you're under there with the measuring tape, if you're planning on 3" tail pipes, remember that the 3" bend is gonna take up some room.