The swirl tank mods are the same thing that I referred to as the self-bleeding mods. They are well worth doing. If you want PWM fan control, you'll need a PWM module but your Holley ECU should be able to control it. Mine does. If you go this route, the fans won't switch on and off, they will speed up and slow down, proportionate to coolant temp. It just makes the fans quieter and less obtrusive. Saves on fan wear and tear and keeps coolant temps better regulated too. The PWM module I used (pic below) is from a 2006 to 2009 Corvette and various Fords from the same years.
Most modern automobiles use PWM to control cooling fans. It tends to keep coolant at a more even temp than fan switches ever could. In the case of my Pantera, the coolant temp behaves like the coolant temp in a modern car. It warms up to about 186 degrees then stays there, rock steady. No more rising and falling. It just sits at 186 degrees (on my EFI ECU's reading). It might drop to 185 or rise to 187 for a few seconds but that's it. On my Veglia gauge, the needle just sits in the same place. That's the benefit of PWM.
Pantera Electronics Radiator Fan Controller also uses PWM but I'm not sure if it would interface with your Holley ECU. Probably not but then again, it really doesn't need to!