I sprayed the floor pan with some stuff like Rhino Liner. It worked but just taking some sheets of sound deadener - X mat or whatever strategically in center of open UN-reinforced panels will cut vibration. As a reference - if you happen to have replaced a headliner (in a car where it has probably not been done before) you should see the original GM sound damping sheets in open areas of roof. Just a note - there is a postage stamp size piece the stuff in your picture next to hole for your seat belt re-tractor. There are some threads on this here on this board I think.
If you are interested I have have some extra I could send you for the price of shipping. I think I have a couple different brands - Eastwood etc.all leftover.
Bob