My old pump gas BB stroker motor in my street Duster ran 7;00 in the 1/8 mile locally with the 3.0 inch exhaust system on it, it had a set of MganFlow 14 inch oval x 7 .0inch high side inlet with center exit straight through pipes mounted behind the rear end between the rear subframes and the rear 1/4 panel.
I remove the complete exhaust system, it had a set of Hedman fenderwell race headers on the 505 C.I. stroker motor with 3.0 inch pipes back to the muffs behind the rear end with the muffs exiting at the rear bumpers and it ran 6:98 in the 1/8 mile. Not enough gain for me to worry about. I thought it was probably the loss of weight that affected the ET and MPH
It would run 9.993 in the 1/4 mile at 134.6 mph corked up with the air cleaner on weighing 3450 Lb. with me in it running on 91 octane non ethanol Oregon swill it was a real hoot to drive on the street and sometimes at the track