I installed a AFE air box on my lb7 and it really made the wistle stand out. Did not really notice anything after the turbo back exhaust. The air box made a difference. Towing heavy at 70mph with the passenger window open you can really hear the high pitched wistle. SOUNDS AWESOME.
Keep in mind LB7's had a fixed vane turbo un like the variable one in the newer models. The 6.0 powerstroke is a different animal all together but at least my LB7 still pulls strong with 110k miles on it and still gets better fuel economy.
