Just to interject here again. The Bassett headers are good headers. In our testing we only saw about 10 to 15HP more with the CMI sport tubes, so unless 10-15HP is magic, I don't think you will see 3-4 mph using CMI's over the Bassetts. Don't get me wrong here the CMI's are great headers and we have seen about 565HP on our 525HP motors with CMI's kit and about 550Hp with the Bassetts. Pick your poison, both are good headers! If hatch clearance is an issue, the Bassetts win that battle by about 2". The CMI's are about 1.25" higher than stock 496 risers and the Bassetts are about an inch lower.
Ray @ Raylar