my buddy has a 00 Silverado 4.3 and his sounds just like my 5.3
He left the cat alone and added a single inlet dual outlet 2 chamber Flowmaster with 2 1/2 side pipes behind the wheels
at idle you can't tell if he has a V-6 or V-8 but under acceleration it buzzes a little but Flowmaster is the way to go and if you still want to go louder take the cat off and remove all the inside crap and reinstall as far as being Legal " Well it still has the cat on it"
now when it comes the the sniffer test good luck. Hope that helps.