rear shackle location

At ride height you want the shackle angled with the lower part farther back. If it's straight up and down or leaning forward it won't work. When you hit bumps the spring flattens out and becomes longer, thus the reason the shackle has to be angled back.

The pinion angle is measured relative to the driveshaft, not the ground. You actually measure the transmission to driveshaft angle first then the driveshaft to rearend yoke to find you working angle. The working angle should be close to zero under a full load (wide open throttle on take off). The type of suspension and power level dictates how much negative pinion angle you need. If your running springs with no traction aid 5-8 degrees negative is correct.

Here's a link to an article on setting pinion angle

http://www.carcraft.com/howto/91758/