I take it you have a 4 cylinder in your chevette. The sad truth is there is no way to make a 4 holer sound like a 6, let alone a V-8. If you are allowed to run them, try some good headers and a good 2 1/4 inch exhaust pipe. Not going to guarentee the sound but it will definately run better. The larger the diameter of the exhaust pipe, the deeper the tone. If I were you, I would concern more with getting as much out of the engine as possible and not worry about the sound. Sound doesnt necessarily equate to power.
I would also recommend running a 36 inch exhaust pipe on your 4 cylinder. That includes the collector length.