http://www.youtube.com/watch?v=I5rPPyYdFUs
I literally just shaved the FSB on my Colt 6920MP-B today. I have a 13.8" Troy Modular Rail Forend-RX coming later this week. I did the whole thing with a Dremel and three cutting disks, plus some Birchwood Casey Permablue. It isn't pretty, but it's going to work fine.
To me, it makes more sense to try cutting your FSB before buying a low profile gas block. If you screw up shaving your FSB, well, then you're gonna have to buy a new one anyways, so you might as well give it a shot.