LaRue is first choice for this type of setup.. Daniel Defense second, Troy third .. and if weight isn't an issue but price is more of one then Midwest Industries. You can send your upper to LaRue and they'll mount the rail for you, several DD rails require a proprietary wrench, but are do-able on your own, Troy I'm not sure about, and MI just requires that you cut the Delta ring and spring off (dremel's rule) ...

DD and Troy can run a bit more than LaRue and as stated, LaRue shwag rulez!