the quarters are probably part of the rear bumber cover, and would probably involve bolting into the current fenders, as well as some rivits and tabs.
you havent taken into consideration the massive amount of modification this kit requires to your car. for instance axle width. if you put that kit on yor mustang without modifying your wheelbase, your wheels are goign to be about a foot from the edge of the fender flare, tucked way back inside the car.
lets not forget the couple hundred pounds of fiberglass your adding. talk about dead weight.
if you want to be all show and no go, then go for it. but get ready to spend alot of money, or get ready to be very disappointed. you will need to pay professionals to do this.