Zalman z9 case configuration

my specs:
intel 2500k stock cooler is used
gigabyte 670 windforce 3x
zalman z9 not the plus one
I will make a very very tiny OCing
PLZ make in consideration that I live in dusty and hot environment

I got 2 problems
1-I will use only 1 additional fan 40 cfm in addition to the front and rear fans that came with the case.I am confused where to place it?


2-I got antec 750 HCG psu with fan on top and the case psu mounting part is grill with no filters it will catch dust like a hoover and afraid of inapropriate cooling (very narrow opening) if inverted the psu to make fans face downwards.I need help with PSU positioning fans up or down?

any help would be appreciated thanks!!!