From the Factory, the 1974-1978 Mustang II came with both a front bumper, and a lower valance. The valance most commonly came in black, and sat below both the bumper and the bottom of the radiator.
Using the Mustang II Spoiler and Valance
In order to mount a Cobra II lower spoiler to the car, a valance is required. The valance itself mounts directly to the lower section of the front quarter panels (pictured below), and beneath the front bumper. In order to install the front spoiler, the front valance is necessary as it provides the structure and support to hold up the spoiler. The spoiler itself can be attached via bolts to maintain the exact contour of the valance, and will remain ridged.
Installing the Valance
As is outlined in the video above, the front valance can be mounted directly to the car using only the two front quarter panels. The valance will need to be trimmed and sanded down to match the exact contour of the car's body. It should be noted that the front quarter panels can be adjusted in and outward by loosening the structural tabs on the inside of the front fender. Once this is properly adjusted, you can mount the valance by drilling 2 small holes in both the quarter panel and the valance. You can then use a small bolt and washer to mount the valance until it is tight against the car.