Its a pain in ways. Best IMHO is to remove rear seat, quarter panel trim and rear quarter windows. Have someone hold the scoops in place ( I would paint the area behind them and the back side black first) the drill the quarter panel and scoop in at least two places along the lip. Then use the holes to rivet the scoop in place. Continue around the edge until done. If you have the Mustang quarter trim it will need to be removed first and the holes filled with chalking or dum-dum.
Scoop quality varied litte since its a fairly simple item
We run a '65 coup at the open track events here in Calif.
Most of us have some sort of front disc brake cooling but
not many have anything on the rear. Most of your braking is
done with the front tires. It is a good idea to put brake cooling
on the car, but for a street driven car the scoops that go on the
side will let in water and could start some rust problems unless you
seal every hole you make very well. I know one Mustanger who added
some to his '67 and in less than one year he had rust in that area.
They look great and do a fine job for a car that can use the air. If you
put them on, take extra time to stop any water from getting inside the car.
Functional is the key word. Simply putting scoops on the rear quarter scallop does not make a functional brake scoop. It takes cutting the quarter panel, the wheel house and fashioning a tube in between. This is fairly simple for racing where the interior is not refitted but more complex when you want to keep the interior. The scoop and tube have to be weatherproof, such that rainwater not a problem and it's best if you figure a way out that separates air/water. Even if you get the cutout and tubing fashioned, there's no assurance of airflow will be directed at the brakes. Cutting up your quarter panels often degrades the value of your car. It costs about $1,000 to restore sides to cars that have been made into Shelby clones with functional side scoops. Be absolutely sure you want to do it.