As I recall there is an adjustment you can make on the switch. I recall there are elongated holes on the switch body where it can be moved back and forth to adjust the switch.
From experience, if your brake switch is orginal or the first generation of reproduction, the shaft that slides back and forth through the switch housing is stuck.
I tried to free mine and purchased a replacement instead.
as has been mentioned, the mounting holes may be slotted to allow for adjustment. Mine had no slots, so I used a rat tail file to create slots for adjustment. Worked for me.
You might care to explain how you tested your original Ford mechanical stop light switch or do you have hydraulic brakes fitted ?. In any case you could just disconnect the wires from the stoplight switch which should cause the lights to go out. If they are still on then there is a wiring problem. Regards, Kevin.