I built a car for a guy and the new water pump he had was bad. The impeller was spinning on the shaft.
It would do the same thing with the slowly rising temperature.
A fan shroud is a must
Also check the WPHousing.. A lot of the aluminum WPH s look OEM but some of the ports narrow down quite considerably (like too a 1/2 inch) on the inside. I put back an old iron OEM WPH and my 383 and it runs cool as a cucumber......
