After owning more than a dozen Mustangs and Resto-modding some and Pro-Touring others, my experience with radiators has been the following:
For OEM fit and superior cooling, Fluidyne. Why? They are all alum and have 3 1" tubes and not 2 1" tubes like the rest. They sell for about $360 at Summit Racing.
If you don't mind trimming the radiator core support, Ron Davis radiators are second to none. He is the same guy that does the radiators for NASCAR race cars. He sells both oem fit and the one I just ordered which needs the trimming done to the rad support. They are pricey but worth it($860 manual trans, add $100 auto) Here is the link and a pic:
http://www.rondavisradiators.com/radgallery.htm
Look at the fit against the rad support and the finish, very nice.