On the recomendation from the guy that built my radiator I mounted a 16" thermo fan. This one is rated at 2700CFM like Chuck's. The radiator guy does lots of work for hot rods and custom stuff with hot motors and he says the 16" thermo fan is plenty to keep pretty much anything cool.
What's important is the shrouding. The flexalite fan may be able pull 4500 CFM of air but how much of it comes through the radiator? These 16" fans are well shrouded and pull almost all their air through the core.
I have one for the warm propane (LPG) powered 351C in my F100 and it keeps everything nice and cool (unless the fuse blows
). LPG makes the motor run a fair bit hotter than regular petrol too.
Cheers