18” or 18.5” fan for a 440?

I figure I could custom make a shroud if I have to.
I just need to know if I need to stay away from the 18.5” fans.
They have one down here at Carlisle for sale for $150 and was wondering if I should pull the trigger?
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.