How stock is the motor other than the GT40 heads? Do you have a full exhaust? Have you changed the upper and lower intakes? Do you have anything other than the stock airbox and piping? Stock throttle body? Auto or manual? What rear gears?
You see where I am going with this? Without any of this basic info and how you intend to drive the car (street, street/strip, track only) and what rpm range you want the power in its like throwing a lawn dart up in the air at a crowded family reunion and expecting it to hit big foot's pinky toe.