This photo shows an '87 GTA rear end with disc brakes. These are the smaller pre-'89 10.5-inch iron caliper units, but they're mounted on a 3.27 limited-slip 9-bolt. If you're working with a 3.08 or higher open differential, you'll see a substantial performance increase from this swap. The '89-and-later disc cars were upgraded to the better 1LE rear brakes. Also pictured are the rear WS6 springs. Drop the shock mounts and they virtually fall into your hands.
Many junkyards charge a core fee for rear ends. If you recoup your core fee by trading in your old rear, it's often much cheaper to buy an entire junkyard limited-slip rear with the gearing that you want than to buy a new limited-slip differential and gears for your present rear-end housing and have them set up.
Keep in mind that when changing gear ratios, you need to swap the corresponding speedometer gears inside the transmission to correct the speedo reading, or it will be wildly inaccurate. To get these parts, follow the speedo cable to the transmission and remove the driven gear housing on the side of the trans. The driven gear will come out with it. Then remove the crossmember bolts and unbolt the trans tailshaft housing to remove the speedo drive gear from the tailshaft.
Installed exclusively on Chevrolet IROC Camaros-but also quite effective on any Third-Gen Firebird-is the "Wonder Bar" or steering brace (highlighted in red), which mounts between the subframe horns and the sway bar brackets. Most that we've seen in junkyards are bent, but substantially stronger versions are available at reasonable prices from Top Down Solutions, Hawks, and BMR Fabrication. Note the IROC's stabilizer bar thickness at 34mm-the WS6 bar is larger.
Give 'Em A Brake
Scoring an entire rear end also provides a great opportunity to upgrade rear drum brakes to discs, or increase the size of your rear disc rotors. The '89-and-up aluminum PBR calipers and 11.655-inch diameter rotors are considered the best (earlier rotors are about 10.5 inches).
Though there is no difference between the drum and disc brake rear-end housings, each rear (9- or 10-bolt) requires different backing plates, brackets and the correct axles for its particular type. As the different caliper designs also require different rotors, axles, and brackets to fit correctly, your best bet is to get the entire assembly, if possible. If upgrading from drums to discs, you will also need the proportioning valve. All rear-brake upgrades require the parking brake cables specific to the new brakes.
Upgrading the critical front brakes using factory parts is tougher, because only the 1LE (factory road-racing package) cars and '89 Turbo Trans Ams used twin-piston calipers on larger 11.86-inch diameter rotors, as opposed to the non-1LE 10.5-inch rotors and single-piston calipers. Unfortunately, the 1LE brakes are a rare find due to low production-only about 3,000 cars total between Firebirds and Camaros were built with this option, primarily Turbo T/As.
Stiffer urethane sway bar bushings were available on performance Third- and Fourth-Gens. The urethane bushings are usually white or brownish-white after years of exposure, and improve sway-bar effectiveness but will not tear and crumble like the rubber units (main image). They can also be purchased new (inset) for about $15 a side with new hardware from Top Down Solutions.
As performance increases quickly overtax the braking on a Third-Gen car, and "boneyard bolt-on" upgrades are extremely scarce, we looked into other options. The cheapest and most expedient was to upgrade the stock front system to HPS pads, slotted rotors and steel braided brake lines, for an improvement in both stopping power and pedal feel. Another option was to purchase a complete 1LE front brake setup from Hawks Third Gen for about $850, but keep in mind that you will need to drill out your stock spindle bolt holes to the larger size for the 1LE brakes. All of the shops we spoke to offered this as a service with a spindle exchange.
An excellent low-cost brake upgrade option offered by Ed Miller involves buying the more widely available C4 Corvette 12x0.81-inch rear rotors and calipers from a junkyard and adapting them to the Third-Gen spindles for use up front. The 12-inch C4 caliper is essentially a slimmer 1LE caliper (same diameter pistons), so the Firebird's stock master cylinder is properly sized and proportioned to handle it. Paired with the '89-and-later aluminum PBR rear calipers and rotors, they're essentially a full 1LE brake setup, provided you find a decent deal on a serviceable set of rotors and calipers. Any of these options will still require adaptor brackets, hubs, and drilling of the stock spindles, so consult Ed Miller about price and availability of parts to bolt them on.
Suspended Sentence
Third-Gen Firebirds have earned a reputation as great handlers, thanks in large part to Pontiac's WS6 suspension package. Standard on GTAs and Formulas, and optional on Trans Ams, it consists for the most part of a quick-ratio steering box, upgraded front and rear sway bars, stiffer front and rear springs-which lowered the car about 1.5 inches-and higher-rate gas shocks/struts. The icing on the cake is larger 16x8-inch wheels and 245/50x16 Goodyear "Gatorback" tires.
Always bring your calipers when shopping for sway bars, as all are not created equal. On the left is a Pontiac WS6 front sway bar, and on the right is a Pontiac WS6 rear sway bar. Compare the front sway bar diameter here to that of the Chevy IROC in the photo detail of the red Wonder Bar in the photo at left. | 
This photo shows a WS6 aluminum driveshaft on a non-junkyard car, so it's clean. When it's been under the car for 20 years, it can be difficult to tell from a steel driveshaft. The aluminum shafts are not magnetic, so if you can't tell by looking, try a magnet on it or scrape a small area to see if it is light (aluminum) in color or dark (steel) before you pull it out. Also, make sure it's not seriously dented anywhere or it may throw off the balance. | 
Large-for-the-day, 16x8-inch wheels came on many later performance-package cars, such as these GTA "Cross Lace" wheels. Formula WS6 wheels are the same dimensions, but a decidedly different style. Keep in mind that there are distinct offset differences for front and rear wheels for Third-Gens, easily seen by the "lip" width. |
A simple way to identify a prospective "roller 350" is by the steel coolant lines (highlighted in red) that run from the passenger-side front of the engine, under the crank pulley, and down the driver side of the oil pan to the oil cooler, which mounts between the block's oil filter boss and the oil filter itself. These lines and cooler are easily transplanted onto a 5.0 (305) so make sure you have a 5.7 (350). | 
A raised "5.7L" or "5.0L" cast into the block behind the driver-side cylinder head (green box shows detail) positively identifies this as a 5.7L, or 350, CID block. | 
Another desirable option that will require some wiring to install on a standard Third-Gen Firebird are the dual fans from a GTA or 350 Formula/IROC Camaro. |