Stock 2010-2014 mustang have a stud sizing of 1/2" dia. x 20mm. You throw a 8mm spacer behind the wheel you just lost 40% of your threads. At least 2 issues I see from the get go, 1st you would most likely strip the threads if you pre-torque the wheels to the required force. If you have not already stripped the threads you just over pretension the bolt to cause failure from any lateral forces. 2nd to have the proper pretension you are still at the issue of 40% of your frictional forces between threads of the bolt and nut are gone so now the nuts are more likely to brake lose with out any thread lockers.