If you take your time and do a neat job of it, there's nothing wrong with trimming down an a2 front sight to make a poor mans low-profile gas block. As mentioned before, it's not the most asthetic option, so it's probably best reserved for use with a free float tube that will completely cover it. There are some inexpensive low profile gas blocks that would work just as well. clamp-on style is probably best if your barrel isn't dimpled for a set screw. Troy does make a longer low-profile gas block that's designed to hide the taper pin cuts in your barrel, but it's definitely not the cheapest option.
For gas block install, brownells makes some plastic pins for aligning the gas ports. They're super easy to use. When you're all done and set, you can just use a cleaning rod to break the pin and let it fall out of the barrel. The other easy way to align a gas block on a disassembled barrel is to plug the chamber end of the barrel and just blow through the muzzle end. You'll know you're aligned when you feel the lest amount of resistance.