If you are setting your mustang up for racing then and don't know much on chassis and rear ends at all or not much then I recommend a couple books that I just got the other day that help a lot. One of them is "Hot Rodders Bible" the other is "So Cal Speed Shops, How to Build a Hot Rod Chassis", those are very helpful and will teach you all about the ladder bars or 4 link or whatever you wan't to do. Well worth the 15 bucks on each book I think. Now I am with everyone, I have done some research on rear ends and a 9 inch sounds good. If you will have about 500 HP then you will need to strengthen up the chassis a bit so it doesn't do what Matt there said....