18” or 18.5” fan for a 440?

If it fits into the shroud you use, then you should be fine. There is not much difference between the two fans. The issue is blade pitch as was mentioned earlier. You want a lot of airflow, and a proper shroud will help focus that air through the radiator. A poor shroud will impede airflow.
IIRC the fan is supposed to be 1/2 inch into the shroud