Jack the car up and put jack stands under the rear portion of the front frame rails. Allow to sit for a day or so. This will allow the car to settle to the correct position. What has happened over the years is the shock towers have collapsed a bit from normal usage and poor planning from Ford. They did not plan on the Mustang lasting so long. lol Now, when you install the brace, I believe you loosly mount the points over the shock towers and you can use a bit of pressure to pry the back bolt holes into aignment.