The 3-row works up here in Colorado - so well, in fact, that I have to block off half of it in the winter (when the temperature looks like my checkbook balance) just to get the thermostat to open!
In the summer, my electric fan never comes on now. It used to come on at most summertime stoplites with the brand new 2-row I first installed, using the FoMoCo flex fan. First I changed it to a clutch fan, which improved the top end HP noticeably as it moved more air at stoplights, but with the A/C on those triple-digit days, it wasn't enough - hence the electric adder.
Now, the electric one is just added weight! I used the 3-row from a 1980 T-Bird with towing package, by Modine. It's a crossflow, so I'm not sure how that will work in an early Mustang.