I had a set of the old Magnaflow 14 inch oval by 7 inch tall 3.0 inch side inlet straight through to a center outlet mufflers with 3.0 inch pipes from the BB Hooker 1 7/8 primary pipes headers with 3.5 inch collectors, the muffs were mounted behind the rear end. It was fairly quiet at part throttle and got louder at WOT shruggy
That combination was .02 ET and .2 MPH slower hooked when up than removed on my old pump gas Duster, 7.01 at 95.2 MPH in the 1/8 mile versus with them removed 6.99 ET at 95.4 MPH, I'm sure that the weight had something to do with those results, not the back pressure form the pipes and muffs twocents

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)