All Steering Wheel Nuts for 1967 Mustangs (and for many years before and after) were Hardware Part Number 350983-S, This is a 5/8-18 nut, which requires a 15/16" wrench or socket. It is a plain(bare steel) finish. It is unique in that a "standard" 5/8-18 nut is 35/64" thick, but the steering wheel nut is only 5/16" thick.
It should be torqued to 20-30 ft-lbs, The Assembly Manual notes to add 2 drops of thread locker after torquing, to the juncture of the threads and nut.
Bob