Camshafts, intake, Holley Carb and headers are the obvious starting points to make a 66 389 perform...but it would help if you could answer a few questions. Are you sure it's a 66 GTO 335 HP 389? What CFM is your Holley Carb? What are the casting numbers on the heads...(numbers are found on either side of the head on the center exhaust port)? What is the mileage on the engine? Is it an automatic or a standard? What is your rear end gear ratio?