Ford 2003 Mustang Workshop Manual page 1155

40. NOTE: The gaskets are reusable if not damaged.
Remove the bolts in the sequence shown, remove the intake manifold and gaskets.
Installation
1. Install the intake manifold and gaskets, tighten the bolts in the sequence shown.
2. NOTE: The O-ring is to be installed on the top of the thermostat.
Install the water thermostat and the O-ring.
Install a new O-ring as necessary.