Yes, that relief cut is necessary for the extractor claw to rotate into. Also on those barrels, the gas port needs to be drilled, the barrel pin (once the barrel is properly headspaced of course) and barrel component retaining pin holes need to be drilled, and the handguard flange notch needs to be cut. It's not a terrible job for someone with a mill (hell, I did it all but the extractor relief cut with nothing but a drill press and a rat tail file) but it would be much easier for you to just buy a new gas block and FSB and install them on your already headspaced barrel. This is, of course, that your barrel is still usable.