Mike,

Using Andy Lennons' method for cooling a cowled engine the inlet for cooling should be equal to the finned area of the cyl and head, the outlet should be 140% of the inlet size. I would look at some of the speed fliers airplanes to get an idea of what the do for their designs, if I recall you are working on a very fast airplane, and there methods may serve you better. Although I think that the method that Andy describes in his book might work better and be more aerodynamic, the weight involved with enclosing an engine entirely may be a problem for you in your application.

One bit of advice any square angles should be avoided this will cause drag, round everything as much as is possible inside and out to get the cleanest flow pattern, in the engine compartment as well as the exterior. I would also "Round Out" the front of your cowl as well going to a teardrop rear configuration using a NACA type of scoop for the exhaust. I would also look at enclosing the fuel lines and needle valves as well, the bulk of the cowl to cover these will be made up for by the cleaner flow over your fuse and wings.

And I am no expert by any means I use the swag method a lot!

Regards
Paul