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). To start, bolt the rear of the
 plate using a flat head bolt that will sit flush in the countersunk hole. Use the hole closest to the
middle of the car to compensate for the offset of the holes in the floor. Below is the driver side
and you can note the inside holes are used. The other rear holes are for the passenger side.

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 installation method will sit the seat lower to
the floor and provide more legroom as well.