You have the option of looking at the RPO codes but dunno if they will be there for one and two the rearend although not likely could be different than what it came with. Pontiacgp is telling ya the most accurate way to do it and I would agree, your fluid probably needs changed anyway. Most likely you got a sh*t gear set in there. 3.23 or 3.42 posi would be the way to go in a street car with a 403 in my opinion. Highly unlikely you have either of those and not a posi so your best bet would be to look for a 7.5 rear end somebody put a posi and a similar gear to those in and buy it cheap. I paid 300 for my last posi 7.5 with aftermarket axles and new brakes with a 4.10 ratio. They are out there. And no a 403 isnt gonna break it either. Just a pre-emtive response for the guys who are gonna tell you that you need a built ford 9in for a 180hp 403 
