I would have had an answer for you, but something happened to Jay's web site. He has the chart that you are looking for. However in looking at your profile did you install a 67-70 9 inch rear in your Mustang? See this chart for the difference between a 65-66 rear and 67-70. fd
I didn't install it; that's the problem. I do know that when I had the center section rebuilt the guy at the shop was pissed because he ordered the wrong bearing set for it; it needed heavy duty bearings. Would this lend a clue?