Get under car and look closly at unibody rails.I have restored many mustangs and this is a budget braker.Rails are available but require jigging car to replace properly.Also look under upper control arms. There is a recess in the rail that collects dirt and debree over the years and causes them to rot out. Look closly at the upper inner fenders.Just forward of the hood hinges, I always find rott here. Good luck.