My main rifle started life as a 516 (I love the lower); and there's nearly no way to get that gas block off of that barrel without butchering it. If you want to save some coin, and get a 300 blackout rifle, I've gotta suggest just buying/building a new upper to go on your 516. If you go the build rout, and want to save a little more, you can just use the same upper (that's what I've done).
In the vein of what smokin said, it's gonna be cheaper to get a new upper or have the whole existing upper rebuilt, than trying to salvage that gas block