Quoted:
You have to first remove the brake from the barrel, looking at your configuration my guess is that its fixed in place and would be a true pain to remove.  That leaves you with tow options, trade in that upper and buy one that already has the rails, or if you really have a lot of free time on your hands you can drive out the two pins holding the front sight in place, take a dremel tool with a cutting disk and carefully cut off the front sight, then you can install the hardgaurds that is if the locking ring they give you will go over your brake.  Then you can replace the front sight that you destroyed, with one that clamps on.  Far easier to just replace the barrel or the complete upper.