Group something else to note is that even in the P51 duct later models had a wing section to help direct the airflow into the top section of the duct. The idea has always been to slow the airflow to take advantage of whatever fin pitch is used and then to pinch the duct again to re-speed the airstream to near to the aircraft speed. Never 100% obviously but as close to it as possible. Now I going to pass along a possibility for cooling that I have told a few people but not many. There are a bunch of conformal radiators available to us for a reasonable cost that I am amazed nobody is using. High-performance motorcycle radiators. I worked for Kawasaki years ago and did a lot of engine building even into the water-cooled era. Several motorcycle manufacturers have built beautiful curved aluminum radiators to increase the cooling available on some of their  highest performance models. I have attached a jpeg of the radiator from a Kawasaki ZX7R. 750cc engine rated at about 110 HP. Very high heat rejection requirements. Lots of these components available from motorcycle salvage yards. often with minor damage to the end tanks, but for our use we might modify them anyway. Fitting one, (or two), of these curved radiators inside a cowl would be easyand effective. You might never get a radiator man to make you a ring radiator, but you can buy some that might conform very well to an in-cowl mounting.

Bill Jepson