I'm not running near what you are power-wise, but I had a flex fan with a 4 core radiator in my 327. In 85 + heat she would heat up relatively fast at standstill (220 in about 10 minutes). I have a good shroud and the fan was spaced correctly in/out of it.
A friend convinced me to put a large mechanical straight fan on it before installing an electric fan. I was sure that this wouldn't make a difference. But the blades were longer, fatter and had a greater pitch. Got it for nothing out of a Ford (!) van in a junkyard and cleaned it up.
So far so good! I let it run in the garage for 40 minutes in 70 degree heat with the hood down. Hasn't got above 190 yet, even in 85 degree weather. When cold I notice the HP drain slightly. When warmed up I don't notice it at all.
The fan type, size, pitch can make all the difference in the world.
68 Camaro ragtop, original 327, angle plug Camel humps, 202's, Performer, Holley 650 double pumper, Doug Thorley headers, 3-speed Saginaw, 3.08 posi, subframe connectors, Addco sway bar