I am really surprised this didn't kick up

I had to skip from page 2 to 6,....and I haven't watched the vid,....but if one stiffens only one part of a structure, there can be a proportionate increase in the flex of the rest of the structure with the same applied load....the "only as strong as the weakest link" law. However, My buddy's 340 dart 4spd started kinking the quarters just below the C pillars with only narrow cheater slicks, and for the lift from the rear loads, which can easily be applied with modern rubber,...streetable or not, the rails need tied......and like Tony, I've been around too,.....and seen a lot of sub-par/sh!tty pinch/spot welds in My time..................