I had a 440 1969 Dart that picked up .8 and 9 MPH by removing a set of 2.5 turbo muffs and sides pipes exiting in front of the rear wheels, I used a set of 14x7x3.0 inch oval Magna flow with three inch pipes from the 3.5 inch Hookerfender well headers on my pump gas Duster. The muffs. are mounted behind the rear end between the rear sub frames and outer 1/4 panels, it is nice to drive and only looses .04 ET and .15 MPH in he 1/8 mile when I've removed the complete exhaust sytem at the local track, probally the weight making the differences and not the flow restrictions, not worth removing for that little gain
The Duster runs low tens(best of 9.993 at 134.5 MPH
) on pump gas, motor only with the air cleaner and exhaust on
3450 lbs with me in it