What is the difference between a 23" fan shroud for a small block with air and a 23" fan shroud for a big block? My parts catalogs show 2 different part #'s. I have a 327 small block-no air-in my car with a 23" 3-core radiator. Which one do i need? Also does anybody have a used-but not cracked-fan shroud 23" for sale? Thanks Jim.