FFR offers three different rear suspension options.
1. The standard FFR kit without a rear suspension upgrade uses the stock 4-link out of a mustang with a solid axle rear-end. The stock 4-link causes a lot of grief when setting the ride height. Also, the 4-link will tend to bind a little in the corners when pushed really hard. You'll get a stiff, harsh ride with a 4-link.
The big advantage to the 4-link is that it is very strong for handling the launches with big motors. If you are on a tight budget, it's one place to cut corners and you can upgrade later. If you want to upgrade the 4 link without going all the way to a 3 link, you can ditch the stock springs and shocks for a good set of coil-overs for around $400.
2. For around $1k, FFR offers a 3-link upgrade that includes everything you need to attach your rear-end to the frame, including the coil-overs.
The three-link is a huge improvement in ride comfort and car handling. You'll get much better cornering response out of the car, yet it will still handle a lot of horsepower. The best bang for the buck upgrade in an FFR is definately the 3-link.
3. For around $2k, you can upgrade the stock FFR kit to include an independent rear suspension. You will need to provide a Ford Thunderbird Super Coupe rear-end (pumpkin, axles, rotors, etc), but you will end up with an excellent ride quality and fantastic cornering abilities. Is it better in the corners than the 3 link? That depends on the driver, really.
The IRS will increase the resale value of your car as well, but it is a more complicated piece of machinery. You will need a rear-end alignment when you get everything put together. From what I understand, and I may be wrong, but the IRS is not as strong as the 4 or 3 link, so you may want to carefully check into how much HP and torque it can really handle.
Finally, there's great FFR information at the FFcobra forums. Check them out at
http://www.ffcobra.com
Enjoy.