Another overheating situation 63 Slant six

Couple of comments. The stat allows pressure to build up in the cooling system [ inside the engine ] because it becomes the restriction to flow. You obstruct the flow of a fluid, you build up pressure. This pressure subdues localised boiling around hotpots, like the exh valve area. Boiling water has air bubbles, & air is a poor conductor of heat. Anything that reduces or eliminates air bubbles aids cooling.

All else being equal, an alum rad cools better than copper brass. Alum allows the use of larger tubes which transfers heat better & the larger tubes also allow more efficient fin design. The fins release the heat to the air. The brass & solder in copper/brass rads are also poor heat conductors.