RE: Convoluted speed theory
My thought is your forward opening should be no bigger than the fin area on the head and cyl. And should be ducted or directed right at the motor, look at Andy Lennon's book on design. Also the flat area on your firewall will cause an area of high pressure reducing the amount of flow through the cowl. You want all of the airflow through the cowl to be focused on the engine filling the cowl with air does nothing for cooling any the air that does not go directly past the motor does nothing for cooling at all.
Ask Da rock or Bmatthews in the aerodynamics forum about it.