Centering a rear end
Just today, I read an article in the Dec 2012 Car Kulture Deluxe magazine that described installing a Nova rear end in a 51 Chevy. Basically, they took the centerline between the frame rails, hung a plumb bob at that point, and lined up the rear end with the bob. They clamped the spring pads in place and postponed welding them until the body was set back on the frame and the ride height was set. This allows the spring perches to be welded in place with the proper 2 degree offset angle for the driveshaft.