Mounting brackets for the 1967-1969 Camaro/Firebird's are available. Below is a picture
of the floor pan under the driver seat. The mounting holes in the floor are just to the left of the "X's".
The rear holes are offset slightly toward the middle of the car.

The plate below will bolt to the floor (shown unpainted for clarity). Initially you will bolt the rear of the
 plate using a flat head bolt that will sit flush in the countersunk hole. Which hole you will use in the
rear will depend on which side of the car you are installing to compensate for the offset in the floor.

Another flat plate will bolt to the underside of the sliders provided with the seats. Flat head bolts will be used
here too since this plate will sit on top of the plate bolted to the floor.

In the front, one bolt will be used to bolt both plates to the floor. In the rear, the top plate will bolt
to the bottom plate which has a threaded hole in it. This will allow the sliders to sit farther back in
the car and increase legroom.

This installation method will sit the seat low to the floor with the sliders being 3/8" off the carpeting.