Jazty, ...sorry! I was readying way to fast and missed the details in your original post!!
You won't believe it, but many manufacturers lack some serious aerodynamic knowledge and/or just approach things with a "a bigger fan will solve that" attitude.
Aerodynamic R&D often is not done in house but contracted out...
There are two old books out there (from and before WW2, FLUID DYNAMIC LIFT and FLUID DYNAMIC DRAG by Hoerner)
[In case you wonder - a lot of performance was to be gained by streamlining the huge cooling needs of the aircraft engines - what applies to them applies to road vehicles just the same, at somewhat lower performance scale]
There is a LOT of formulas and drawings in there - ...just looking at the drawings and analyzing the comparisons you could solve a LOT of the cooling problems and even improve on efficiency!
Have a closer look: Most fan arrangements do NOT seal to the front of the radiator - a big deal of forced airflow escapes to the sides before going through the radiator.
A lot of time, there is not enough space behind the radiator for the air to escape freely!
I bet instead of huge louvers are small line of vortex-trips in front of the intake and possibly a small lip at the rear could change the pressure in front of the radiator dramatically....
AS mentioned the floor is open in the engine room, maybe even to the rear, but what about UP??
Hot air loves to rise!! WHY force it any other way??
You don't want huge louvers on the side - you will not always drive at full speed - what about trundling along a mountain road or in traffic and having a strong tailwind - effective ram-air could become ZERO in a heartbeat.
Some experimenting along the lines published in autospeed.com can show you fairly quickly where the different pressure areas on your bus are.
After that it would be fairly easy to start to work in the right direction......