You never want to alter the size of the exhaust headers, manifolds, or up-pipes on a turbo-diesel. That's what builds the pressure in your turbo. Running a 7" stack is just a matter of running a straight-pipe 4 or 5" turbo-back exhaust to the elbow of the stack kit. running 7" stacks, and you're looking for the most sound, 4" down pipe, and a 5" kit will be what you're looking for.
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