Bax,
I also have about 3" from the waterpump pully to the radiator (4 core Aluminum from JBGY). I have a $250.00 Black Magic S fan that won't fit. It is about 3.5".
JBGY sells a custom 7 Blade fan belt driven - (17") that is designed to pull air through a 3 or 4 core radiator. I ordered this fan and a shroud. I wanted electric, but I just couldn't find one that was thin enough and could pull enough air. I'll let you know how the fan and shroud work out when they come in later this week.