Shouldn't need any. I am told the fan blade 18s will clear the Brembos. Of course I didn't know that when I used them as an excuse to get Shelby Razors... my fan blades are under my workbench and I plan to put them on when I send my rear Shelby's off to Weldcraft to have them widened. Then the cat's out of the bag...
Test fit and see if there is any rub while the car is jacked up. My understanding is that they will fit. If they don't, I have a set of 20" chrome Shelby Redlines for sale that they'll fit under