Dont even know why I'm still attempting to help, but here goes. The "stress" was already there, and when you tacked it, instead of taking steps to counter it, you increased it. The problem is two-fold. First off, as you were tacking, you went too fast between tacks, and didnt let the heat come out enough between welds, so by the time you got to the end, the panels were pushing away from each other, widening the gap you were spot welding. The clamps wont prevent this from happening. You can actually watch it happen if you are taking your time, after a few tacks, you can sight down the gap between the two panels, and see it opening up along the panel. If you stop welding and wait a bit, and as the heat comes out, you can actually watch the gap close itself up again. If you just keep throwing down more tacks you will lock the ever widening gap in place. Also, as others have said, you should have hammer and dollied the tacks/weld seam as you worked across the door, this will counter the tendency of the crown in the panel to pull in, and it will also FORCE you to slow down on the tacks. When you are welding sheet metal like this, you arent just distorting it in one direction, you are distorting it in both planes at the same time, if you dont take steps to counter that, you end up with a fucked up panel.