Jack your Mustang up and use jack stands to gain ample working room under the car.
Disconnect the negative battery cable.
With the age of the Fox Mustang, it might be best to spray exhaust manifold bolts and cat back bolts with penetrating oil to ease removal.
Oxygen Sensor Removal: From under the car, disconnect the oxygen sensor electrical connections from the main wiring harness.
1979-1985 Carborated - There will not be oxygen sensors so this step will be skipped.
1984-1993 Fuel Injected - There will be two oxygen sensors that will need to be disconnected
Disconnect the midpipe to cat back bolts. Slide the cat back away from the midpipe to disengage and support the cat back lead pipes. If you let the cat back hang, the tailpipes can come in contact with the rear bumper and may damage the rear bumper.
Support the midpipe and disconnect the four midpipe to exhaust manifold/header bolts. This will allow the midpipe to be removed or pulled away from the manifolds. Note: If a smog system is still installed, disconnection of the smog pump tube may be required.
Remove any strut tower bars if equipped
Air Intake System Removal:
1979-1985 - Remove any air cleaner tube(s) from the air cleaner assemblies.
1986-1993 - Disconnect the mass air sensor electrical connection (if equipped) and remove the air intake system from the throttle body and fender.
Disconnect the spark plug wires. Mark the spark plug wires if they are not numbered so you can reconnect them back correctly.
Remove the spark plugs.
Remove the manifold to cylinder head nuts/bolts. Once all the retaining hardware is removed, the exhaust manifold can be removed from the top of the engine. (Header removal and installation is done at the top of the engine compartment)
Clean any gasket material from the cylinder heads. Use caution to prevent material from entering the spark plug holes. TIP: You can plug the exhaust ports with rags and plug the spark plug holes with the previously removed spark plugs to seal the holes if you need to clean gasket material from the cylinder head.
Replace the exhaust manifold gasket and reinstall the manifolds or shorty headers. TIP: This is a great time to replace your cylinder head to manifold hardware with fresh hardware.
Restall all removed components. Reverse the previous steps to complete the installation.
Exhaust Torque Specs:
The below specs are based on stock components (cylinder heads, spark plugs, manifolds, exhaust, etc.) and are used as a reference. Always follow the manufacturer part recommendations if different from those below. After a couple of heat cycles, it is best to recheck torque specs.
Oxygen Sensor - 30-36 lb-ft
Spark Plug - 5-10 lb-ft
Header/Exhaust Manifold to cylinder head - 18-24 lb-ft
This is a general installation guide; some Fox Body shorty headers may require additional modifications depending on the brand. Always follow header manufacturer instructions.
Always check all fitment notes on recommended additional parts and notes about your year Mustang.