(1) Too much clearance between the water pump impeller fins & divider plate.
(2) Ignition timing too low. A good vac advance, that will add the correct amount of advance at idle & cruise rpm, can reduce temp.
(3) Fan not pulling enuff air, for some reason. There are some 6 & 7 blade fans that are designed to pull more air. If a clutch fan, their are some HD fan clutches that will make the fan pull more air. Many say that the fan blades should be partially in & partially out of the shroud.
Assuming that your rad is flowing enuff coolant, & that your thermostat is working correctly. Some say that an alum rad, with at least 2 rows of at least 1" wide tubes will cool better than a stock type 4-row, which has smaller tubes. Some disagree. Some also recommend drilling some small bypass holes in the thermostat. Most recommend a 180° thermo. I always used a 160°, or, in high air temps, just a gutted thermo.
