I'm often surprised at the number of guys who will slag you for cutting a pinned FSB down while preaching the use of a set-screw or clamp-on block. It's a critical point in the whole AR system and the pinned FSB isn't going to let you down!
I have used clamp blocks with no issues and the set-screw block is fine if the barrel is dimpled.
My pics are hosted on Photobucket and I copy and paste the image link into my reply.