73 Dart Sport Seat Belt Bolts

Are you talking tunnel, or rocker?

There should be a heavy metal plate under the floor-pan on the tunnel side, and the rocker should have a threaded anchor built in. At the rocker side, there is a hole in the floor pan so the bolt can pass through and into the rocker. The flexing that you are seeing may just be the floor pan tin. The real strength is in the rocker anchor. Just my thoughts without pics.