If you have the funds now is a perfect time to upgrade your axles, assuming you will continue to modify your mustang and have plans for more power. I just broke my stock axles last month at the track and I upgraded to 31 spline and a locker rear end. I stayed with 4 lugs for now as I do not have enough dough for the new wheels but I could have done that as well. My stock axles did not last too long on my car. Moser has a website to help you determine what spline for your power goals http://www.moserengineering.com/moser_interior/axletech.htm