What Was the Original Angle?
What was the angle on the original rear-end that was in the car? I don't know off-hand what it was, but if you do not have the original rear, I would find one somewhere and measure it and copy it on the 9-inch. You can easily get a magnetic base protractor for under $10 to measure this. GM engineers spent a lot of time and money doing R&D to get those angles...I'd try to keep them. I'm sure someone on this board can quote the angle off the top of their head, but I'm at the office right now, and don't have the foggiest.
Sorry I couldn't be more help.