Set the FCS II boxes in flat, the cant is built in but may have to be adjusted slightly to 5 degrees if there is a concave or Vee in the tail. It doesn't really matter though. 5 degree cant is just a standard number and your board will surf fine if it's not exactly 5 degrees.
No need to use Cab-O-Sil when installing FCS II, FCS Fusion, or Futures fin boxes. They are a tight fit in the routed pocket and straight resin is all you need.