I can put 2 x 140mm or 3 x 120mm on each side. This is what the middle 120mm would look like (toothpicks and scotch tape to the rescue) ...
That should give me good air flow straight over the mobo and into both HDD cages.
Here is where a 140mm fan would go (my thanks to the 120mm standing in when required) ...
Offset air channels that take air from the gaps coming from the 120s. I am thinking of having a 3x120mm bank for inflow and a 2x140 bank for outflow.
I can also put a 140mm at the front with a bit of case modding.