Yes you do. The synchronizer is the causal part to the DTC P0340. The common cause is for sludge to build up on the shaft of the synchronizer, restricting free movement. Usual symptoms for this problem include, engine running rough or stalling intermittently, squeaking sound similar to a belt squeel etc. This part has to be timed during installation..
Usually on the vehicles I work on, the cam position sensor is usually destroyed by a syncronizer that has gone bad. Think of it as the lower end of a distributor .. that's all it is with a sensor instead of a rotor and cap on top.
It's pretty much a Ford only item as far as I know and Rakel is right about it needing to be timed or check engine light and fuel economy/power issues will possibly result.
This is also only for the 3.8 mustang motor, the 5.0 uses a sensor too for the 94-95? model year if I remember (either that or used a distributor)
The 4.6 is plugged into a hole in the front cover and doesn't need one.