[rattus]

The body, subframes, etc. all have factory specified dimensional relationships that are independent of suspension components. These dimensions (available in a GM repair manual) should be checked with subframe rails and rocker panels level (front to back and side to side as depicted in factory manual diagrams) and corrected if off by > 3/8" before the cage is welded into place. This ensures that everything is square and in the correct relation to everything else. In other words you want the subframes and body to all be level and within factory spec. as you weld in the cage. This way the cage will help everything stay where it is supposed to be.