I have a 534 and actually the 'venturi' duct fits an 80mm fan, though a fan is not required since it is supposed to line up with the CPU fan. What you can do is place the fan between the case grille and the duct.
I had an 80mm fan installed on the duct in my case to improve airflow to the CPU HSF and found that it worked quite well. The duct is adjustable in height, but if you have a large HSF, you still may not have the clearance. Also, you must have a CPU fan that mounts facing the duct rather than perpendicular in order to make the duct functional.