Chevrolet: running hot, redline water wetter, chevelle malibu

Question
QUESTION: My husband and I purchased a 1965 chevelle malibu.  When driving in warm temperatures the gauge goes to hot.  We replaced the thermistat and that did not fix the problem.  We removed the radiator (2 core)and brought it to see if it is flowing properly, it is fine.  What else could it be?
ANSWER: Has the engine been modified? If its putting out more horsepower than stock it might need a little something more in the cooling dept in way of electric fans or redline water wetter with 100% water and no coolant in the rad for the summer months.

Or your heater core could be plugged.

---------- FOLLOW-UP ----------

QUESTION: The engine is a stock 327 and we replaced the heater core this past winter.  Do you think we need a 3 or 4 core radiator?  Also, we know the radiator is not original, because the fan shroud that we purchased does not line up properly.
Thanks for your help

Answer
Then its possible the previous owner cheaped out and bought a 2 core rad. Still, the engine shouldn't overheat with a 2 core. It should run a little warmer but not go into the red.

3 core rad would be decent but I would even suspect that the rad cap is not correct either. If you have a faulty rad cap or the wrong one your car could overheat.