I have CRS so I have to write stuff down. This is what I did to my '68. Sorry the cut & paste from my build notes came over kind of funky.
Please note, I'm running an iron head big block Chevy, no A/C, aluminum manifold, headers, 700r4. (all weight counts, went through several front spring swaps and one rear spring swap to get what I wanted.)

Notes
Moog front springs are relatively cheap - $62 today from Summit for a pair of Moog 6312's. I toyed with changing to Moog 6308 but decided to leave well enough alone.
X2 what USMC Ordy said - If you want the car lower in the back look to aftermarket. Global West and Hotchkiss make great lowering springs. Mine are 1 1/2" drop but I might go 2" if I had to do all over again.
I recommend Polyurethane bushings in the rear spring shackle. Using the softer rubber bushing in the front spring eye will transmit much less road harshness to the body of the car. I strongly recommend the use of the GM spring cushion pads. Much better differential housing to spring locating than aftermarket Polyurethane spring pads. Still available from GM.
I decided to save some money and use KYB gas shocks. These are very harsh shocks. Recommend staying away from them. Someday I'll buy some really good shocks - right after I change the control arms. The springs are the muscle, shocks are the brains.

Hope this helps.

Build Notes:

Stock Front Suspension Rebuild:
• Moog K5250 Upper Control Arm Kit – Shaft & Bushings
• Moog K6076 Lower Control Arm Bushings
• Moog K5108 Upper Ball Joint
• Moog K5103 Lower Ball Joint
• Addco 1 1/8” diameter front sway bar Part No. ACO-599
• KYB Gas Shocks Part No. KG4515
• Moog #6312 Coil Spring ’67-’69 Firebird 400 – shortened 1 coil. Tried Moog #5246, 400 convertible w/AC but too tall.

• 6312 Spring Specifications:
o ID 3.68”
o Bar Diameter .64”
o Spring Rate 337 lbs. per inch
o Load 1800
o Installed Height 10.75”
o Free Length 16.06”
o Spring Type Constant Rate

Pozzi Racing recommends a much heavier spring rate, especially for a big block equipped car. They recommend Moog #6082 however the spring rate is very high at 722 lbs. per inch. They also recommend Moog #6308, which is a cross reference to the original Z-28 spring. It has a spring rate of 380 lbs. per inch. Might be a viable option with a little heavier rate as current springs are 337 lbs. per inch.

• Moog 6308 Spring Specifications:
o ID 3.68”
o Bar Diameter .64”
o Spring Rate 380 lbs. per inch
o Load 1607
o Installed Height 10.75”
o Free Length 14.98”
o Spring Type Constant Rate

• Moog 6082 Spring Specifications:
o ID 3.68”
o Bar Diameter .75”
o Spring Rate 722 lbs. per inch
o Load 2406
o Installed Height 10.0”
o Free Length 13.18”
o Spring Type Constant Rate – one end pigtail must cut off

Stock GM Alignment Specifications:
• Caster +1/2 deg
• Camber + 1/4 to 1/2 deg
• Toe-in 1/8" to 1/4" (Radial tires need less toe-in, use 1/8" or less for them)

David's Pozzi Racing Alignment Recommendation (Google Pozzi Racing)
• Caster 5 deg positive, or as much positive as you can achieve up to 5 deg, can use .5 deg additional positive on the passenger side to compensate for road crown. A common setting would be: LF 5 deg positive, RF 5.5 deg positive.
• Camber -.25 degree for mostly freeway driving,
• -.5 for street and frequent hard cornering or mountain roads,
• -1 deg for street, Autocross, and Open Track, this will wear the inner tire tread if you drive a lot of freeways. I use -1.5 deg quite often on cars that we frequently autox and do only a little street driving. you can use even more if tire temps show outside edge of tread is hotter than inside after a track run. If the inner edges of tread show excessive wear, use less negative camber. An autox only car would use -2 to -3 degrees. If you have not done the Guldstrand mod, you can be more aggressive with negative camber and increase negative camber by another -.25 to -.5 degrees.
• Toe in, 1/16" to 1/8".


Differential & Rear Suspension Components
• Ring and Pinion: Richmond Gear #4900961; 3:73:1, 4 Series Carrier
• Bearings, Seals: Richmond Ring and Pinion Installation Kit #8310191
• Axles: Superior Axle and Gear Axle Kit #EV12-2; Direct fit 1541 carbon steel 30 spline axle kit complete with 7/16" press in studs, Torrington axle bearings #DB-67309 and National Oil Seals #86605
• Rear Axle Bearing: BAC #5707 (alternate number)
• Rear Axle Seal: National #86605
• Wheel Studs: Moser #8002; 1/2-20 x 2" long screw in studs
• Cover Gasket: Fel-Pro #RDS55029 (Autozone)
• Leaf Spring: Hotchkiss 1.5" drop spring kit # 2407-C (Front & rear bushings, rear shackles, heavy duty 1/2" U-bolts)
• Tee Bolts: NPD #C-7529-1A (4 pieces)
• Traction Bars: Competition Engineering J Bolt Traction Bars #2101 (Replacement J bolts Competition Engineering #7032 - 2 per card)
• Traction Bar Bump Stop: Energy Suspension #9-9132G (3/8" tall)
• Leaf Spring Cushion: GM #3930052 (Schepel Buick)
• Leaf Spring Bushings: Rear: Energy Suspension Polyurethane #3-2101G; Front: Rubber bushings supplied with Hotchkiss leaf springs
• Front Spring Pocket Mounting: Front J Nuts Rick's Camaro #RC-469; Mounting Bolt Set Rick's Camaro #RC-470
• Rear Shock Absorber: KYB Gas #KG5521
• Gear Lube: Valvoline 80w-90 High Performance Gear Oil, Limited Slip #VV831 (Synthetic lube not recommended with factory clutch plate style gear carrier)
• Gear Lube Additive: GM Limited Slip Axle Lubricant Additive #1052358 (4 oz. bottle)
• Driveshaft: Custom built 3" diameter by .083" wall steel Performer Series by Driveshaft Specialists, San Antonio, Texas. Universal joints 3110. Transmission slip yoke #6081X, 3110/27 spline.

Differential Installation
Competition Engineering traction bars – use GM spring cushions only. GM cushions have a steel locator ring molded into the cushion. Locate rear end to leaf spring much better than aftermarket polyurethane cushions. GM part number 3930052. Still locally available from GM.
To install, follow Competition Engineering installation instructions. Competition Engineering’s J bolds are really soft – follow torque specifications and use anti-seize or risk damaging threads. Evenly tighten J bolts to 50 Ft. Lbs. then tighten Tee bolts to 50 Ft. Lbs. Once all fasteners are at 50 Ft. Lbs., torque rear nuts of J bolt to 70 Ft. Lbs. Competition Engineering’s J bolts come with (4) Ny-loc nuts. Used an additional (6) grade 8 SAE flat washers and (4) grade 8 nuts per side.
Due to lower curb height of Hotchkiss rear leaf springs, original tall traction bar bump stops must be replaced to gain recommended bump stop to spring eye gap of 1/4". Use Energy Suspension Polyurethane 3/8" tall bump stops.
Competition Engineering traction bars must have locator holes bushed. Factory hole in bar is 1” diameter. Used 1”od x ¾” id bronze bushing, 7/16” long. Work well with GM cushions.

Wheels & Tires
Front: (Includes Spare)
BF Goodrich g-Force Super Sport All Season Radial; 225/45R17 mounted on American Racing Wheel Torq Thrust M Anthracite W/Machined Lip. 17" x 7" x 4" Back Space
Tire Part # 245WR7GFASXL-26483; Wheel Part #AR105M7761AML

Rear:
BF Goodrich g-Force Super Sport All Season Radial; 255/45R17 mounted on American Racing Wheel Torq Thrust M Anthracite W/Machined Lip. 17" x 8" x 4.5" Back Space
Tire Part # 545WR7GFAS-11671; Wheel Part #AR105M7861AML