The Mustang II front and rear bumpers have the mandatory safety features that were present in the 70's. The "safety bumpers" needed to collapse in at roughly 5 mph, thus requiring shock absorbers behind each bumper. The stock Mustang II (front and rear) bumpers are a 2 piece design, consisting of an inner structure (stock crossmember) and an outer urethane bumper that you see. Our "bolt on" bumpers are made to bolt directly to the crossmember, which connects directly to the impact absorbers (pictured below).
The Impact Absorbers
As you can see in the above picture, the impact absorbers are small shocks that are meant to compress upon a low speed impact. The steel crossmember (or bumper inner structure) bolts directly onto the impact absorbers by using 4 bolts, 2 on each side of the car. These absorbers leave room for vertical adjustment for the new bumper.
The "bolt on" bumpers that can be purchased through Classic Auto Reproductions come with mounting studs that attach the bumper directly to the crossmember. As you can see in the picture above, there are 4 mounting studs that attach the steel structure to the bumper skin. These are able to move horizontally about 1/2 inch in either direction to accommodate for variations in width (stock cars cam from the factory with a little bit of variation).
The above image shows the inner structure attached to the car without the bumper skin