I have no exprience with the trans coolers but I'll tell you what I know about blocking radiators. First, plan on cutting a hole in the middle to cool the fan clutch. If you don';t it will run al the time once the truck warms up and it makes a lot of noises as well as robbing power. Second, if you're going to block it, block it all. Uneven air flow across the fan blades causes stresses that may break your fan or lead to serious damage. A hole in the middle is fine but don't try to just block half.
-Scott