If you took a socket with a long extension, would you be able remove all of the front bolts holding the gear carrier into the rear end housing? If so, it is an 8 inch rear. On a 9 inch rear, you would have to remove 2 of the bottom bolts with a wrench since the cast housing shrouds 2 of those bolts. An easy way to tell the difference
A 9 in is stronger,get one out of a Mustang or Cougar,it should bolt in perfectly,the Fairlanes and Torinos are to wide.