I'm pretty sure 1985 was the first year for a true dual-exhaust system for the 5.0 Mustang, and the last year for a carburetor. If your car has true dual exhausts, they're aftermarket. Moe than likely, unless you live in California, the cats were removed when the dual exhausts were installed. My car was 100% original, with the original 4 cats. No doubt it would have been louder, had the cats been removed.
You might get more responses if you post on a Mustang forum.