Better than stock water pump, high-flow thermostat, 70% water to AF coolant ratio, some Red Line Water Wetter...
www.stewartcomponents.com
I had a 400 for many years in my 83 Z28, with the stock cooling system, with minimal problems. I had to put my original L69 (well, sort of) back into the car recently to pass California emissions inspection, but the 400 always stayed at a reasonable temp even when I was living in a much hotter part of the country. I had a Stewart Stage 2 water pump on my car for the last couple of years the 400 was in it.
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"So many Mustangs, so little time..."
ICON Motorsports