Winoman gave you EXCELLENT advice, also FantaZ.
Take the time to catalog those screws, fasteners and other things that are missing, broken, marred, or just plain old needing replacement. Then either get them from a bone yard, MotorSports, Victoria British or a good body shop supply house.
The rivets that FantaZ mentions are notorious for losing the little plastic spreader pin. You can buy new ones, but if you take care you should be able to find the pin and re-insert. Or if you are obsessive about reusing the originals, figure out some way of pulling them out via the front. Whether by using a pin, glue, or drilling and tapping a real small hole then using a screw, and yes you can find screws that small check your local radio control hobby shop.
Also take care when removing the vinyl from your transmission hump and the rocker panels. If you do it right there's no reason you can't clean it up and reinstall the same pieces.
Once you have the new floor pans in, make sure they get protected both inside and out, then get some sound absorbing mat material, "Dynamat" is excellent but expensive. Check Eastwood they have a couple types, so does Motorsport. Someone has reported having used POR-15, another the pick-up bed liner spray on stuff. Either way, you want some sound absorbing material there, otherwise you will regret it.
Once the interior panels are out, if you are going that far, take the time to spray some sort of rust inhibiting paint or coating on those areas of the steel that are not normally protected well. I'm talking of the inside of your doors, kick panel areas, rocker panels, behind the plastic pieces etc. You won't regret this.