i had flowmaster super 40's on a 2007 ford 5.4 and with the intake it would resonate in the cab so bad i removed the super 40's and put on 50 series just my exprience and it was at about the same rpm i believe you have a exhaust problem not intake just my two cents.
can't go wrong with a doo or cat support your local dealer it paid off for me they gave me a new engine when i was 3 months off warranty....so i bought another one from them that day.
I have had 3 6.0 for company trucks and all would have brankrupted me stay away unless you got a spare cummins to drop in the frame rails and then you got a truck.
i already have an 08 the rebuilt motor from ford is 26,000 or from the city is 9,000 i am a mechanic by trade and have no desire to take him up on offer but if you are interested i can put you in contact with him
haha all in fun 7g's yep i got a email from someone on kijiji wondering if i would take a 2008 f-350 with a blown motor with a thousand bucks on trade thats a different kind of trade