[David Pozzi]

Each shock has a manufacturers recommended installed length/height.
Shocks with limited travel have less ability to vary ride height without topping out or bottoming, so I prefer a shock with 5.5" to 6" travel on the rear if possible. If I want to raise or lower the rear 1" the longer shocks will allow that. If buying a rear suspension kit, the mfr will have a suggested shock length at ride height. Just make sure his design ride height is close to what you want to run your car at.
OR:
Set your car at it's desired ride height and pick a shock that fits that space. Assuming you have shock mounts in place to measure from.

Once you know the shock length at ride height you can pick a coil to fit it. Estimate the load on the coil and how much it will compress and pick a length that will still allow adjustment room up or down. A first or second gen Camaro will have about 800 to 850 lbs on the rear wheels. The weight of the wheels, tires, brakes, and axle housing are all un-sprung weight so subtract that. It's about 175 lbs per side on the rear, so 850 minus 175 = 675 lbs load on the coil. A 300 lb/inch coil will compress 2.25" so a 10" coil will compress to 7.75". 300lb/inch divided by 650 = 2.25" compression, 10" - 2.25" = 7.75". There are shock motion ratios that throw that calculation off, but on a leaf spring, 3 or 4 link suspension, it won't change much. I like to run a 10" coil and jack up the spring perch a bit to set ride height. Then to remove the spring, I can take the shock loose and unscrew the spring perch enough to get the upper spring seat off without having to compress it with a tool.