Outlet/inlet in this instance it doesnt realy matter, all the air that passes through the fan has to be either sucked or blown through the slots in the side/s.
In this pic if the purple has les area than the blue then you would probably be better off spacing your fan off from the heatsink like the red sketch and alow the fan to blow air over the heatsink as opposed to the green sketch which is what I belive you meant, flowing air through the channels.
I would use thermal epoxy to attach the heatsink to the case.