Check out this link, you need a flat and level surface, put your car on blocks/jack stands that are of the some height (get the tires off the ground!!!) and measure the land marks on the frame to the floor.
http://www.stevescamaroparts.com/Cam...me__Dimen.html
Car could be bent, because of an accident and poor frame/body repair. And it could be out of level because of bad/collapsed springs, two different size tires (different manufacturers of the same size tire have slightly different diameters), frame sagging, (body rot, broken structural welds). Look at the frame bushings (collapsed, worn or loose attachments) and the frame bushing mounts (for distortion in the sheet metal around the mounts, or distortion of the mount on the frame). Depends on the history of the car, but a thorough inspection should reveal a lot of those issues, and possible fixes. Some can be easy and cheap, others, well, not-so-much.
Also, the tolerances on the diagram aren't SUPER critical, a couple of the landmarks on my car are tweaked (at some point in my car's history it took an off-road run) so pay attention to some of those things that show up on the car described above. I found that all the landmarks on my first gen are within 1/4" of each other.
As far as adjustable rear suspensions and aftermarket front end a-arms/sub frame adjustability, look really hard at design and make sure that they have the adjustability YOU want. A well designed aftermarket replacement a-arm system with adjustable coil-overs for frame/front suspension will be adjustable and can correct minor problems, but anything more than minor damage to sub frame can't be corrected with adjustable a-arms/shocks and will cause problems. As far as the rear frame rails, Art Morrison makes replacement first gen rails for their 3-link and 4-link systems. Chris Alston has a replacement rear frame rail system for second gen Camaros that is for their 4-link.
http://cachassisworks.com/cac_70-81C...ameSystem.html
http://www.artmorrison.com/imagelib/..._Rear_Clip.pdf