There is this link from the Pershing Idea page. These appear to be in a fixed position.
http://www.persh.org/Pickup/Third-Row.htm
This thread shows them mounted on square tubing and retains the use of the bench slider.
http://www.67-72chevytrucks.com/vboa...d.php?t=617403
The seat bottoms are short, so if your tall, it may not be as supportive as you'd like.
Using the second link as a model, I removed the clamping/cable hardware (weight savings) and bolted the seats directly to the square tubing, using the existing holes already in the seat frame. This places the height of the seat bottom around 11-12" from the floor pan.
In my suburban, I had to move the seat bracket mounting about 4" to the rear and the seat track adjusted all the way to the rear. I can still slide it forward a bit if need be. Not sure if that's an option in your regular cab.
The cables are easily separated from the levers and clamp and can be tucked into the spring bottoms. With the seat locked upright, all it takes is a slight tug on the appropriate cable to fold the seat forward.