I just finished putting an ADDCO rear bar on my 79 Nova. It should be similar to yours. To locate where to drill the holes for the brackets, I installed the bar in position on the axle housing (being careful to have it properly centered), attached the links to the bar and to the bracket then swiveled it up to the frame rail to mark the holes for drilling. I tried to keep the links vertical. This method seemed to work fine. Hope this makes sense. Sorry no pictures.
Mike
Mike