The C4 switch is where Bruce says it is but according to CBTN, no 69 351W's came with a C4. If the tranny is original, you have an FMX and the FMX neutral/safety switch is under the shift selector bezel.
It will be under the shifter plate. Once the console is removed you need to
remove the shifter handle by loosening the grub screw on stem of handle
remove 4 phillips head screws holding shifter bezel and remove bezel by sliding up the shifter
remove 2 screws holding black plastic piece under shifter pointer and remove (be careful not to break pointer
the safety switch is the cream plastic piece.
You can adjust by loosening the two screws, putting car in neutral and moving switch to where the car will start and tighten.
If the car will start in any gear then the black plastic bit under the switch that fits in the shifter is probably broken and you need a new switch
Had a quuick scroll through on-line catalogues and it doesn't look like it is serviced. Not sure if you can adapt C4 or C6 switch to FMX. I'll check Ford in Oz to see if it is still serviced for our FMX equipped Falcons