When water boils (212 degrees at sea level), it turns to vapor and expands rapidly. The only place for it to go is out the overflow pipe. That's why the overflow pipe is there. If you did not have an overflow pipe, boiling water would crack your block every time.
Many in this forum have reported overheating problems with 235 engines. Search on the word "overheat" and you will find a ton of information.
I have my own theory on this, which you probably won't find using the search tool. Start by measuring the size of your fan. I believe the original fan from a 1949 216 engine is about 16.5-inches in diameter. About the same time GM switched to full pressure engines (1954), they also increased the size of the fan. The fan on my '59 235 is 19.0 inches in diameter. Now, this might not sound like a big difference, but consider this. A fan with a diameter of 19.0 inches has 33% more surface area than a 16.5-inch fan. What's more, as the diameter of the fan increases, the tips of the fan blades move faster at any given rpm. The tips of the blades move 15% faster on a 19-inch fan compared to a 16.5-inch fan at any given rpm. The bottom line is, the 19-inch fan pushes a LOT more air, probably around 50% more air when you combine surface area and blade speed.
The bolt pattern on the larger blades also changed. The short-shaft pumps that I've seen are designed for the smaller 16.5-inch blade. I installed the 19-inch blade on a short shaft pump, but I had to drill new bolt holes in the blade.
With the larger blade, a 180-degree thermostat, and a 7 psi radiator cap, my 235 will idle indefinitely at 185 degrees, even when the outside air temperature is 110 degrees. I can't speak to the effects of the dual carbs, but I imagine more fuel translates to higher engine temperature. If you are using a 16.5-inch blade, you will never push enough air to keep the engine cool.
