I just added a duct tape grill block on the Dakota, it covers 1/3 of the open area. Did this for two reasons, to see if it would help with the FE and to get some heat in the truck. T-stat stuck part-way open, but in order to change it I would have to move the A/C compressor and just didn't feel like doing that last night. It already accomplished one of its goals, I have good heat. Stay tuned!
If you check out the grill block I did on 'Global Warmer', you can see that I covered well over half the radiator inlet area (probably closer to 2/3). I couldn't make any real conclusions about improvment on FE, but it still provided adequate cooling even on 80F days while hauling 1/2 ton of yard materials around. You could probably get away with blocking off at least that much if not more until the weather warms up.