I believe my fan is 19.5" diameter with 6 blades. As to why your shroud is scraping the bottom of your shroud, I have no real idea other than the obvious "either the fan is too low or the shroud is to high" thought. Possibilities that cross my mind to check would be to make sure your motor mounts are OK, or make sure your radiator/shound is mounted or seated properly.
Sorry I can't be more help.