The "Shelby (Arning) drop" is a method for correcting your front end geometry.
In the 60s, insurance companies began cracking down on hot rodding, and didn't want teens driving dangerously in all the high-powered cars on the streets. One way the manufacturers tried to create 'safety' was ensuring that their cars would understeer.
Oversteer is fun. You go into a corner too fast, hang the tail out, rear tires howling, and it's a hoot, right?
But understeer? In a situation where you're going too fast into a corner, the car tends to plow straight ahead and doesn't make the turn. It's not fun, and after one scary skid into a ditch, most kids learned to drive more sedately.
In a Mustang or Cougar, (in fact most Fords of this era) have the upper control arm at a fairly steep angle compared to the lower control arm. When the suspension compresses, the top of the tire swings out, the bottom rolls in, and the outside tire in a turn rolls over onto its sidewall, or at least lifts most of its inner tread off the ground. Instant understeer.
Carrol Shelby, when asked to make the "Secretary's car" (Mustang) into something meaner, decided with Klaus Arning to fix this very simply. He moved the upper control arm down by drilling new bolt holes. The upper control arm, now more parallel the lower one, helps keep the tread firmly planted on the ground, possibly even gaining just a little camber in a turn to brace against it.
It does lower the nose about 5/8" to 3/4", typically, but it's not nearly as much about stance as it is about improving handling, and reducing body roll as well. Combined with a stiffer-than-stock front swaybar and a few other minor bits, the factory suspension on these cars lets them handle very well indeed. Instead of wallowing around corners, leaning over like a yacht with the tires howling, it will just do what it's supposed to. Best of all, the Shelby drop is essentially free!
Aftermarket upper control arms are available with roller spring perches that really free up the front suspension, and many of them come with built-in geometry that allows you to use the original holes in your shock tower, instead of drilling new ones with a template. StreetOrTrack
is a great place to look for a set, if you're interested.