In Ford's four decades of producing Mustangs, nearly all of them have come equipped with a live-axle rearend. Not that there's anything wrong with a straight axle-it's sturdy, inexpensive, and hooks up better in straight-line acceleration. So why even consider switching to an independent rear suspension?
Good question. The answer lies in who's asking, and the intended use of the car. That's because while a live axle performs well on smooth pavement, an IRS handles better on bumpy, twisty roads. Dragstrip diggers are happier with a live axle for its antisquat traits, but an IRS offers better compliance on uneven pavement, minimizing axlehop. It also helps the chassis squirt quicker out of a tight turn when you pour on the power.
If you do most of your driving on rougher pavement where you need more flex in the suspension to keep the tires in contact with the road, then an IRS is the way to go. Not only that, if you plan to do any competition events such as a road course or the new sport of drifting, again the IRS should provide superior handling and controllability.
Reduced unsprung weight is another point in favor of an IRS. How so? Not only is a live-axle heavier, but it usually has more unsprung weight, meaning the weight of the suspension parts is between the ground and the springs. Picture it this way: If you stand on top of one of your car's tires, the suspension does not move at all. That's because you're putting weight on the tire and not a spring.
By doing so, you have raised the car's unsprung weight, so every time the suspension has to conform to a bump or irregularity in the road, the entire mass of the rear axle has to move up, over, and then back down. This large movement of mass can jolt the entire chassis, upsetting the tires that may have had a good grip. If you're on irregular pavement when this happens, it causes loose and unpredictable handling.
The Control Freak IRS conversion includes a custom cradle along with control arms, coilovers, halfshafts with CV joints, and a new differential. Stainless Steel Brakes' calipers and rotors are now part of the package as well.
All of which leads us to the IRS install shown here on a '67 Mustang. Larry Weiner of Performance West had already substantially modified this fastback in a number of other areas, such as replacing the factory engine with a late-model, 4.6-liter mod motor, and supercharging it with a Kenne Bell blower. So Weiner, a builder of numerous show cars enhanced with aftermarket components, wasn't reluctant to try something new.
In this case, the IRS upgrade came courtesy of Control Freak, a division of Blue Moon Motorsports. Headed by Al Kamhi, the company manufactures tubular control arms and IRS systems for late-model Mustangs and older GM vehicles as well. The system shown here fits '6411/42-'70 Mustang model years. Control Freak also now offers tubular front control arms for classic Mustangs, too.
In addition to a triangulated tubular cradle that reinforces the frame and supports the Ford 8.8-inch center section, the system includes subframe connectors to reinforce the chassis. The differential housing does not include the gears (so you can pick the ratio you prefer), but it does have a stud girdle for main bearings.
This IRS package, costing $9,400, also features Stainless Steel Brakes rotors and calipers, along with custom racing halfshafts designed to handle as much as 800 horses at the rearend. That's due in part to the use of competition-grade, six-part CV bearings and cryogenically heat-treated CV housings and race-quality, bolt-in axles. The coilover shocks are QA1, manually adjustable units. All of which means this modernized Mustang should ride and handle quite a bit differently than Ford's original live-axle design.
These are the IRS components. Note the triangular subframe that not only carries the differential and control arms, but reinforces the chassis as well. | 
The first step is to drop out the stock live axle with leaf springs. Even though this unit is simple and sturdy, it doesn't provide as much compliance on uneven pavement as an independent rearend. | 
Next, install the forward end of the cradle that holds the differential and control arms. The cradle bolts right to the framerails in the stock leaf-spring mounting points. |
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With the live axle removed, the framerails first need to be notched and boxed to make room for the control arms. The patch panels are included with the kit. |  |