Frame connectors US Cartool need help.

On Chop, Cut & Rebuild they are doing a Cuda for Goodmark. The program I watched over the weekend had a segment on installing frame connectors. It appeared they did it on a lift supported under the frame rails without the car in drivable configuration. Supporting under the frame rails makes more sense to me then on the wheels. With the car sitting on the wheels you cannot get true level since the springs will dictate the stance and could tweak the car. I just cut the floor out of a 4 door by removing the spot welds. In the course of removing it, 2-3 days, the left rear tire went flat. You could see the misalignment where the welds were removed. I think the car should be supported under the frame rails and held as level as possible, front to back and side to side. Again, I think, my opinion.