Try use stock exhaust w/ a new cat should do.
Fiero exhaust pipes are so short. Shorter and likely same size the car that engine came from.
New monolithic cats have so little restriction engine won't see much if any back pressure from have a cat.
But if not tune the engine and run lean or rich, burning oil, etc, will destroy them fast.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave