I'm late to the party but if you haven't checked out Cooling Components do so.
coolingcomponentsinc.us.com
They offer fans with shrouds for AD trucks. They are thin (2 5/8"). The fan is quiet (brushless motor) and powerful. They are also 2 speed which is nice. There tech line is fantastic and they stand behind there product. I know first hand.
Worth the money. I too went with a cheap Ebay fan to start with but Cooling Components cured my cooling issues.
Thanks
Marc