i looked into this once and researched into putting the car in a wind tunnel,i found a company that does some work for f1 cars,ended up being £500 an hour!but the m.d. was very helpful and had a lot of time for me.
cutting a long story short he said putting the rad anywhere other than the front of the car takes the ram effect away,so i explained to him i was prepared to have any kind/position of vents to make this work,problem is whether its an external or internal vent the air will always want to go around it meaning the air that does go down will effectively stop at the rad.
i opted out of this in the end but possible solutions include running fans all the time or creating some sort of scoop under the car to basically draw the air through the rad,although air underneath has a turbulance effect so trial n error would be needed.
the only advantage to this conversion is the increased water volume!