Jeff,
I agree with Bill. I first thought looking at that is that the air gets pinched off right near the front. I assume you had an issue with clearance to the engine and that is why it got built that way? If so, maybe drop down the leading edge of the radiator a couple inches to make for better clearance before you build up the next duct/diffuser. You want the portion over the Radiator to be more of a true wedge shape, but more important than the pinch at the back you mention is opening up the front.
The other major things to consider are boundry layer, turbulence behind the prop, and AOA. In my plane AOA makes a huge difference. So much so that low power slow flight (below 80 kts or so) does not cool as well (noticablly so) as anyting above 90 kts. The difference is not the lower RAM force of the air, because that is more than offset of the power reduction. The difference is that the increased AOA causes my inlet to spill much more air, and my design is relatively tolerant of AOA changes compared to a more tunnel-like inlet like yours.
The exit can be a pure wedge as well. Easy to make with sheet aluminum rather than trying to get all curvy with fiberglass. My inlet diffuser is a simple aluminum wedge and it works great. (picture attached)
Paul Lamar's book on cooling is probably a very good primer. Although I have never read it Paul really does understand a lot about airflow dynamics.
Good luck, I so not envy all the fiberglass work you are about to embark upon.
Dave Leonard