I've never seen a reference to a rear sway bar on first gen firebirds, but there was a rear sway bar developed for camaros that was listed in the heavy duty part supplements of the 68-72 dealers parts catalogs. It was developed during the 67 trans am racing season but was never put on a production car. It looked identical to a first gen camaro front sway bar but was mounted backwards and upside down. The end links were attached to brackets sandwiched between the leaf spring pack and the axle. Not many were used because starting in 68, the shocks were staggered in front and behind the axle which interfered with the bracketry. Estimates are that possibly as few as 10 sets were ever produced. Can you imagine what one would be worth?