Verify you have the correct water pump.
Serpentine pumps generally spin opposite of a V - belt
due to the belt route.
There are water pumps that are made to spin in the conventional direction internally when used in a serpentine system.

Verify the fans PULL air through from the rad towards the motor, and they are NOT pushing toward the front of the
car. The hot air will just hang around and not get evacuated.

Take out a few sparkplugs & see if they are fouled in some way.

Basic but a lot of times overlooked --
Verify the cooling system is burped of air bubbles that may be trapped.

I don't know if 2600 cfm is enough for a Big Block.
Or is that 2600 EACH?
Anyone have any thought on cfm for those fans on a Big Block?