How do I find out info on 69 el camino G.M heritage center dosn't go back past 1977?

I have the 69 el camino that came either from the factory or the Dortwhiler dealership in El Monte Calif. with a dana 4/11 posi rear end. I have orig. P.O. which states it was ordered with a posi rear end, but not what kind. G.M. Corp. refered me to The G.M. Heritage Center but their archives only date back to 1977. it was first purchesed by grandfather in Oct. 1968. I've been around the car all it's life and the rear end has never I repete never been out of the car. the dealership is no longer in business. So all you car buffs where do I go to get rock solid info. on this car and yes I've heard all about how this can and can't be possible. I do know cars and this is whats in this car.