Manifold Heater for Cold Climates

Before getting concerned over being able to turn off the hot water to the carb base, I would install the intake and hot water box and run it.
My guess is the long runners on the AS intakes will like a warm carb base to keep the fuel in suspension at all times.
Hot water heat on an intake will max out 190 to 210 depending upon the thermostat. That is much less than the radiant heat that will be hitting the sides of the intake runners from the header flanges.