the main benefit of each is increasing horsepower by removing back pressure in the exhaust system. Yes you can probably get them for a V6. The glass packs willdo the most for the sound, although the headers will change the sound of your engine as well. Headers are one of the first things people recommend to boost horsepower, and a higher flow muffler is probably the first thing to do about sound.
A V6 firebird is generally going to be thought of as underpowered, so if you are looking at prestige, be careful about spending too much money on that engine. You'll never be cool in the gearhead circles running a V6 in a car that can handle a V8