Steveman, Adding two Pioneer 4" speakersin the front is a good idea if your want a center channel for your sound system, but heres something that might help.
This is something I did in a few Impalas Ive worked on over the years. All of the vehicles were non A/C cars.
1st are the parts needed, A pair of 6x9 adapter plates, the one I used are for the sail pan on 1982 to 1992 Camaros. You can use Plastic but the metal flat plates are the best.
2nd youll need a pair 6x9 Foam Speaker baffles (to keep the speakers dry) Plus they act the same as a enclose Box.
3rd, A good pair of 6x9, 2 way, 3 way what ever you want. Maybe a 100mf non-polarized cap if you run an amp to keep the speaker from playing to much bass.
What I did was remove the complete vent assembly at the kick panel, Clean and trim the area of shape edges. Lay the 6x9 adapter on the vent hole and trim the adapter to fit, so it will fit flat up against the vent hole. Then Install using self tapping screws. Next install the speaker wire and cap (if used) make a small pin hole threw the speaker baffle, install the speaker and baffle assembly onto the 6x9 plate install your kick panels, hook up the wires to the Amp or Head unit and thats it your done.
You will get some nice front fill this way, but no more vents
I have pictures but for some reason I cant post using the pics as soon as I find out how I will
However here are the item numbers for some of the parts needed
Hope it helps
Baffles on eBay, item number: 253051928889 Camaro Plates eBay item number: 372131208467