Reading all your other comments and no reading this, it seems your grille opening is just not big enough to feed your radiator.
Just like how 69 Dodge Daytona's tend to overheat at idle and low speeds. Same cooling system everthing is on a 69 Dodge Charger that does not overheat. The Daytona just has a smaller grille opening.
Open up this grille squares. Maybe redirect the brake duct cutouts to feed the air box. And you liceanse plate is blocking the main grille opening. Doesn't seem like much, but in your case everthing counts. This would be a start.