FWIW... My mild 350 doesn't overheat with the stock 2-core radiator, even w/temps over 100*. And the stock clutch fan sits
several inches away from the radiator (but flush with the shroud). However...
Several years ago the fan clutch went bad, so I put on a flex fan which was a perfect fit. Only thing is, the truck would overheat! So I fixed the clutch (new silicon fluid) and it's been good ever since.
Obviously, my opinion is ditch the flex fan and get a clutch fan. You should be able to get a junkyard unit cheap. Just take measurements, look for a relatively new one on a larger car or truck. And oh yeah - throw a thermostat in there!
