Oh WOW that looks pretty sweet!! Nice work Rob. I wonder how the rear spoiler would actually mount and flow with the rear of the 69 Riv? The pic of the rear end is an old one I saved from the for sale ad when I bought the car.
I think you would be in for alot of work to make it fit. Anything is possible but the center of the rearend sticks out more than a 70 GS. Plus the quarter panels are very different than a 70. I had to skew the hell out of the spoiler just for that photo. If I can find a view of my GS from the back I will try that one for you