What you should do is put a fan shroud on it.
should solve all of your cooling troubles.
I believe the fan is supposed to sit half way into the shroud.
if you have a shroud that is 6" deep your fan should be in about 3"
If you dont like the look of a fan shroud, ditch your fan and get a good electric fan.
When you run hot in stop and go traffic or ideling but runs cool at 35mph +. (and your air/fuel mixture is correct) It means your fan isnt pulling enough air through your radiator
If you just switch to an alum. radiator, you'll most likely have the same problem, and have no more money in your pocket.
Ditto the shroud. Mine did similar things until I located the proper shroud for thr car. Tracked one down used from a used parts vendor out of Mustang Monthly magazine... don't remember who....