I don't think I'd even bother using the radiator cooler with a fin/plate cooler with a fan!!!! I've run the fan cooler by itself on many cars and trucks, even a tow rig or two, with absolutely no problems.....

Will you be mounting it with the fan towards the trunk floor, or the fins toward the floor???? With the fan towards the floor, it should be able to draw plenty of air....with the fins toward the floor, I don't think an inch would be adequate.....

I also usually add in a stone deflector in front of the fan, just a little L bracket the width of the cooler, to the assembly....