[dalola]

Rick,
You may want to verify that temp. with something other than the factory gauge, if you haven't already. Those factory gauges, and sending units, are usually not very accurate.

If in fact things are as you say, the condition you describe sounds like either a capacity problem, or a fan problem. You say the fan clutch is "fresh", meaning new? Doesn't garantee a good part. Double check that. Try heating it up manually to make sure it locks. Also, is there a possibility you have an air pocket in the system? If you recently drained and filled the system, are you certain the correct amount went back in? Beyond this, not sure what else it could be in a basically stock vehicle. I've had many 5.0 Mustangs through the years, never experienced any overheating problems if everything was in order.
Good luck,
Dave