I'm doing metal work on my 70 Challenger and I was surprised to see that the floor boards really don't have much in the way of support underneath. The only support the get cross ways is the transmission cross member. Aside from that, the floor rests on the rockers and a few small side supports. There's a little added strength where it's welded to the back floor pans but that's not much. It doesn't surprise me to hear that the center area near the hump would sag over time. Especially if you have heavy people dropping themselves into the car.
If your not wanting to do any kind of floor work and you don't like the US Car tools sub frame connectors, I think shims on the inside seat tracks are really the only fix.