If it is not the one that came in the car, then you are miles ahead, because that weak 6 banger rear end cannot handle the V8 torque. Hey, this site is designed to talk about Mustangs. We enjoy the discussions, so don't apologize for asking questions. Now, go to the classic section on here, and at the top you will see a FAQ section. I have gathered a ton of information that will help answer most of your questions so you don't have to surf. I have it listed in various sections, so go to the differential section and you will see the information you are looking for. If it is a 9" rear, it will have the hump, but if it is an 8" rear it will be round.