Cooling systems without a water pump rely on the difference in density between hot and cold water to create circulation by natural convection. Just like air, as water gets hot it becomes less dense and will rise. So inside the engine's water jacket, the coolant is heated and tries to rise, being replaced with cooler water from the bottom of the radiator. The hot water moves into the top of the radiator and is cooled there, becoming more dense and tending to sink to the bottom of the radiator. This is called the "thermosypon" effect.
Higher concentrations of anti-freeze impede the ability of the coolant to circulate. I live in western MD where temperatures drop down to the single digits in the winter, and I still use only 25% antifreeze, 75% distilled water in my YM1110D which doesn't have a water pump. I have gone through 3 winters without freezing the block. If I were you, I would use the 25% anti-freeze/75% distilled water solution and not worry about freezing. It is also important that the cooling system be very clean.
You should start by using a cooling system flush product to flush all the gunk out of the radiator and block. Use a hose to back-flush the radiator and the engine block. You might want to remove the radiator and take it to a radiator shop for cleaning and pressure testing. Then fill with the 25/75 mixture. You should have no overheating problems after all this.
