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I would agree with Rob or Iraqgunz.
Ditch the upper first.
If that's not an option, ditch the rail and front sight he has now and go with a good rail that will fit over the gas block and either an LMT fixed rail mount front sight or a Troy folding rail mount front sight. I prefer fixed sights, and would personally go that direction.
Last option would be to send it to ADCO to have the gas block replaced with a standard F marked FSB. This service is listed on their website for $90. Of course their will be shipping either way, etc.
I have used ADCO a couple of times in the past and always had excellent service, a good price, and very quick turn around.
-RD62
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Actually a pinned front sight base with the A frame removed makes a good low profile gas block. If it is the type pictured, just grind off the sling swivel tab and the bayonet lug, contour the bottom edges and paint it. Now install a free float rail and front sight and you are set. Or for a low cost solution get the Bushmaster modular accessory system sight that is designed to bolt on over the gas block (part #yhm-9360k).