As RRR said, pump speed is very important at idle. I think the heavy duty cooling option had the water pump turning at 120% of the crank speed. Think mine has a 7.25 crank pulley & a 5.75 water pump pulley to get this ratio.
A shroud would help a lot, just make sure fan is 1/2 in, 1/2 out of the shroud. I learned that the hard way.
