Pretty sure you're going to be needing a .750 sized gas block. Where'd you get the Bushmaster barrel from? I would call them to ask what size the gas block journal is.
I have not seen a 450 barrel with a .936 journal, so it leaves the other 2 sizes mentioned previously.
I used a Satern barrel. It has a Mid-length gas system with a .875" inch journal. I used an SLR fixed gas block, a Tromix compensator, and Tromix modified Lancer magazines with the Tromix aluminum magazine follower. Carbine weight buffer from Aero Precision. The barrel came with a fitted bolt, and I installed that into an AIM Surplus EXO BCG I had. Shoots real nice, but I am am contemplating going to an A2 fixed stock just because I don't really need the adjustability a carbine stock has.