Holley 3 bolt carbs like Y blocks and 302 GMC used were probably the worst about icing. We always ran a little over rich mixture to get longer life out of our valves. Of course Zenith did it too. It was always in fog or high humidity and 38 to 40 degrees seemed to be the window of icing. However, in Texas, colder than that usually means a blue norther with cold dry air.
