Ford 2003 Mustang Workshop Manual page 2919

30. Remove the extension housing.
31. NOTE: If a new countershaft extension bearing was installed, install a new bearing cup.
Using the recorded end play measurement, select and install the appropriate shims to achieve
the proper end play. Install the countershaft extension bearing cup.
Lubricate the bearing cup with petroleum jelly.
32. Install the fifth/sixth shift fork snap ring.
33. Using the special tool, press the fifth/sixth driven gear.
Align the splines of the fifth/sixth driven gear with the drive gear on the countershaft
extension.