Well actually you have a couple of angles to worry about. Those spring perches...just make sure they follow the same line/angle of your springs. Then there is the driveshaft angle which is more critical. Driveshafts work best with angles that are similar at both the differential and transmission. And the angle can only be so many degrees. This driveshaft angle stuff ties in with the installation of the perches...you got to have those driveline angles right before you weld. I found a lot of stuff on the internet simply by using a search engine such as Altavista...simply type in "driveshaft". If you can't find what you are looking for I have internet reference material at home from when I changed out my rear axle.