I looked it up in my old Walker book & there are no B-Body dual exhausts showing a rear resonator in any diagrams from 1968-1974, even though I know some models came with the 4" diameter resonators. The '71 GTX should have the frame mounted brackets with the rubber isolator that the chrome tip hooks into. You can use this along with a [ bracket & a 4" band clamp to grab the resonator. Check out this photo - this was the method I used to grab the resonators on my old '73 Charger.