You'll need a new h or x pipe with any header that isn't modeled after the stock manifolds.

My mother had the BBK longtubes on her Cobra. These were definately some of the highest quality headers I had seen at the time (circa 1999). She went with a D&D k-member at the same time to make installation and clearance a bit better, but this wasn't absolutely necessary to wedge those suckers in there.