X2 what 79 said
Had a catted 81 J bodied Cordoba with a /6 and the exhaust was holier than a tramps socks. We had no testing back in the 80s here so I just ordered up a front pipe for a 78 Caravalle, which in Canada never came with cats. Pure bolt in and $19.00 if memory serves.
I know it's unbelievable if you're coming from a Ford, but some parts from 20-30 years before still fit the newer bodies. Heck the Chrysler section of the Walker catalog used to be paradise for those of us wanting to retrofit to cat-free. They even used to offer oversized Y-pipes. I bought a 75' B-body off a mechanic buddy and he'd ordered an oversized Y-pipe and installed it. MASSIVE ! It fitted a 2 1/2" muffler, but "necked down" to the 2 1/2" muffler !!!!!!!!!!!!!!!! It was sewer pipe sized and could sweep the road shoulder when you tipped in the secondaries.