can i use 65 and 67 headers on my 66 pontiac

Best advice I can give you is to forget about headers on 64-67 Pontiacs. First, you will see almost zero gain in HP as the 389 and 400 engines make their best torque and HP at lower RPM, where headers have no effect at all. Second, they are pure torture to install, leak, and will give you ground clearance issues. Instead, get yourself a set of aftermakrket Ram Air 3 manifolds and 2.5" dual exhaust. They will produce extra HP across the entire RPM range, look great, take the heat off the engine, and last a lifetime. And save you $$ besides.