Well I finally got an exhaust for my 04 Denali xl 6.0! Gibson 615534 from summit racing. 409 stainless cat back dual in with a single side exit. It was tough to get all the clearances right with the axle and coil spring but we got it to work. Cold starts are awesome! Great idle tone, great burble. Acceleration is louder than stock with a very nice tone, a little aggressive. Not loud enough to piss off the neighbors. There is a little resonance (not quite a drone) at around 2000 rpm 65-73 mph that even my wife finds tolerable. With the K&n intake and this exhaust this truck is much more fun to drive. I’ll report back if I notice any mpg increase. Cheers!