To form the flat receiver you need a real press brake, a box brake may not have enough pressure and some one who knows what they are doing or you will go through several flats before it's usable. You should also bend the narrow front and rear first so you have room to get tooling into the receiver for the 2 long bends.
Measure the distance to the bottom of a complete receiver, then subtract 1/3 material thickness, this is your bend allowance, or how much the material will stretch during forming.
The Trunnions must be absolutely straight when welded or you might as well just throw the whole receiver away.