Step by step process of an UCA drop on a classic Mustang, Cougar, Falcon
I grabbed some of the verbiage below but be sure to check out this link there is tons of information that I can't duplicate here in a message board post.

A Little History:
- The Upper Control Arm, or UCA drop AKA Shelby Drop, or Arning drop, is one of the more beneficial and easily the most cost effective suspension upgrade that can be made to a 1965 to 1970 Mustang (also works for Falcons and Cougars). This simple act of lowering the mounting point of the upper control arm does more for the way a classic Mustang handles than many expensive aftermarket components, and the best part is, other than the cost of your time, the modification is free, or almost free. Most refer to this performance upgrade as the Shelby Drop. This is due to the fact that the only Mustangs that arrived on the showroom floor with this modification were the 1965 and early 1966 Shelby models. The UCA drop was just one of the many modifications made by Shelby and his crew to these Mustangs. Even though most refer to this modification as the Shelby drop, in all actuality, if this modification were to be more accurately named we would call it the Arning Drop. Klaus Arning was a Ford suspension engineer who designed this modification to go with a rear IRS system he had designed for the early Mustang. Ford decided against the better camber UCA location on the production line, however Arning was able to pass the info for the UCA relocation on to Shelby, who incorporated it into his cars.
- Most likely the reason ford located the UCA where they did was to create a camber curve that lent it self to understeer. Understeer is a situation where due to slight traction loss in the front wheels the car does not corner to a degree of sharpness that the angle of the tires would suggest. It is common practice among automobile manufacturers to configure production cars deliberately to have a slight understeer. If a car understeers slightly, it tends to be more stable (with drivers of less ability) if a violent change of direction occurs, thus improving safety. This does not mean the UCA drop is unsafe, it just means if you are going to make the steering more responsive and you need to drive the car with that in mind.
That is a great link!
I did the Shelby drop on a 65 Mustang that we owned many, many years ago. Took the picture we all see floating around the internet on a piece of paper to a Xerox machine to get the 1" measurement dead on to scale then took it home and used spray adhesive on a thick slab of steel to stick the paper to it and make a template. Used 1/8" pilot holes for the new drop locations. I figured if they are requiring you to be 1/8" rearward then you'd better be dead on so that is why I made the template to be dead on. Bolted the template in place (Had to wrap the bolts in tape to make sure they were centered in the existing factory holes as they were a tad bit too large). Drilled new locations and then made them bigger with a bigger bit. Stopped at 1/2" bit as the UCA went right into those holes perfectly (didn't have to go up to a 17/32" bit like was suggested many times).
Also boxed the LCA's, had a HD Moog ball joint installed on the UCA to better handle the angle imposed by the drop and had 1 degree of caster added to the UCA. John Dinkel at Open Tracker Racing did this for me. Great guy to work with and uber knowledgable on setups.
I added a heavier sway bar, installed a ORP roller idler arm, Baer tracker rod ends, installed a dual master cylinder and did the Granada front brake conversion and made it power brakes as well. That '65 handled like a go-kart. Simply incredible. Well worth all of the mods.
I did the 1" UCA drop while restoring my Mach 1. It was simple. I made a template using the info in the Boss 302 Chassis Modification Guide. I also had only 1/8" pilot holes in the template for the new mounting location, then removed the template and drilled to the final size. Simple, excellent, inexpensive modification.

At the same time I welded in place the 14 piece shock tower reinforcement kit to help ensure no shock tower cracks from the higher rate 600 lb./in. coil springs.

I've read elsewhere about the original UCA location being intentional to create understeer. I don't know if I embrace that as being the entire reason.[/align]

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