I was taught that when welding rigid frame pieces to a unibody, the car should be supporting its own weight on the suspension with the doors shut. Of course, that's assuming that everything is square to begin with.
Not sure if that's right or wrong, but that's how I've always done it and my cars go straight down Broadway. And my doors shut like they should.
Here's when I did mine. Race Ramp blocks so I had room to work, and the post lift held the connectors in place so I could weld everything
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