Ford 2003 Mustang Workshop Manual page 2743

4. Install the rear countershaft bearing race.
5. NOTE: Do not install the shims at this time.
Install the countershaft rear bearing retainer.
6. Using the special tool, measure the countershaft cluster gear end play.
If the end play is not within the specifications, remove the countershaft rear bearing
retainer, and install the necessary thickness shim(s). Reinstall the countershaft rear
bearing retainer, and recheck the end play.
Bend the tabs of the countershaft rear bearing retainer over the bolts when the end play
adjustment is complete.
7. Position the reverse idler gear and bushing in the case with the shift lever groove facing the rear
of the case. Install the reverse idler gear shaft and the reverse gear overtravel stop.