I installed a fold down rear seat in my 69 SS coupe. I used all the information I could find on this site and on Firebird forums. My suggestion is to us as much GM parts as possible - including the brackets that get attached to the floor - if not I guess repro will have to do. There is a difference between the 67 vs. 68/69 units. There is a hand-drawn map somewhere on this site that has all the measurements you'll need - they were spot on.
Measure three times before you do anything and good luck.