I have a 67 390 mustang and want to beef its power output. I have heard the exhaust manifolds are a weak link. Are 428CJ heads a better upgrade, or can I get what I need from the GT heads with some modification to them? Likewise, should 428CJ manifolds replace the GT units? Or could I Extrude Hone the GT units? I want the engine to look factory(no headers). I have a 428PI intake to add. My goal is to add 50 to 75 horsepower and significant torque above the stock engine.