Bill,
You need to size the opening to the required engine air flow at your maximum speed and then I would make it about 10-15% larger then you need to expand the volume of the inlet tract after the inlet to reduce the speed of the incoming air. By reducing the air velocity you will increase the pressure at the engine inlet, which is just what you want. By doing this you can take advantage of the "ram effect" of the high speed, and don't believe anyone that says it is worth 3-4 psi at 200 mph, properly done it may be 1 psi but that is still a 6% improvement.
Rex