As close as possible really. 3/4" should still pull some thru the core though air will still take the path of least resistance which ever that may be. Some fan kits offer a cable tie system of mounting thru the core , which is the closest you can get but I feel puts stress on the core. Simple mounting brackets fabricated on the sides tho hold the fan assembly 1/8" or so from the core offer by far the best air flow efficiency . How is your fan operated, by manual switch or thermo switch?
