Ernie, I think there is more going on here than we understand. As you can see from my avatar, my Arctic grille cover blanks out more of the radiator than your plywood cover, but I am still running the stock fan and my engine temp seldom gets above 180 degrees, even in 85+ degrees ambient. What I supect is happening is that in my case, there still is a lot of air flow through my ENTIRE radiator. I suspect that excess air is driven through the radiator from the "air dam" effect, that is the forward motion of my vehicle creates a high pressure across the front of my truck, forcing air under the front bumper and upwards behind the grille cover. The stock fan is at the same time creating a negative pressure, which pulls the excess air through the entire radiator, so I am still cooling water in the entire radiiator, albiet at a reduced air flow rate.
I think you are correct in your assumption the plywood is blocking too much air flow through the radiator. You are only cooling water in the 16" diameter space directly behind your fan (roughly 200 square inches). Even though my air flow rate is decreased, I am still flowing air (with the aid of my stock fan) across approximately 576 square inches of radiator, so you have effectively reduced your radiator to 1/3 of the original size. I suspect opening up your plywood cover to expose more of the radiator will fix the high temperature problem.