There should be a flat (horizontal) mounting plate on the driver's side between the grille and radiator. I don't see one in your picture; perhaps it's still in the parts pile? The shot below is of a #28 in a 1964 S/C combination.
To activate the siren, there should be a Ford-style starter solenoid on the driver's side fender well that's wired to the siren/horn switch.
Are you available tomorrow afternoon? I'll be in Enid in the morning, and could possibly run up there after lunch. I just sent my contact information via PM.