Carb icing happens most often with ambient temperatures around 40 degrees F, and the venturi action along with fuel vaporization causes the throttles to become cold enough for humidity to freeze on them. Eventually you get enough frost to kill the engine at idle. If temperatures are much over 45 degrees, the throttle plates won't get cold enough to freeze the humidity. Much under 35 and there isn't enough humidity to freeze.
You need to keep the throttle body above 32 degrees F. At first blush, that's not all that warm, therefore it shouldn't be too difficult.
Realistically, though, running coolant through the crossover passage is unlikely to do much for you because the heated coolant is so far from the throttle body.
Now, if you were to build a carb spacer with a water jacket, that'd be a whole 'nother story.
Pulling the manual choke halfway out until the engine warms up will assure that in addition to carb freezing, you'll also drool enough fuel down the manifold to wash the oil off the cylinders. Not the best plan.
If this were me, I'd leave the manifold alone, and run a stock-type thermostatically-controlled air cleaner and heat stove on the exhaust, so you could pre-heat the AIR going into the carb, instead of heating the manifold or carb itself. And when you want performance, kill the vacuum to the air cleaner so it pulls in outside air. Problems solved, no plumbing nightmare, no welding, fewer hassles, and the manifold stays as cool as possible for best performance when you need it.
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