Do you know for certain that the unit you have is correct for the car? There are (were) different types, depending on the year. The earlier coil/ignition units were slightly smaller, that is, the cable was slimmer, the lock cylinder part was also smaller. The "tab" with which you screwed it into the underside of the dashboard was different, with the earliest types extending back about 2 inches along the cable, and the later types sort of folded under in a "U", so it mounted closer to the keyhole in the dash. Look to see where the 2 side-by-side holes on the cylinder are, and compare them with the holes under the dashboard.
Hope this helps.