Bob is right about the flat spots on 69 axle tubes. In the 69 facelift of the car they dropped the ride height a half inch with the springs and used the flat spots to retain the same suspension compression travel as the 7 and 8s. You did not mention what 9 rear you have, standard or traction lock, 4 Pinion Nodular or standard 2 pinion grey iron and what gears? Also why the rebuild just because or did you check it out? Check out these pages to get you started with identifying what you have. https://www.firebird400ho.com/nodular-rear-end-date-codes/ and http://wildaboutcarsonline.com/memb...ents/9990382806291/Pontiac_Diff_Codes.pd note that there is a mistake in the second one as the 3:36 and above traction lock rears are 4 Pinion just like the 68s.
