You need to proceed VERY carefully. If you bought the lower in your original post,you are then in possession of an unregistered short barrel rifle, looking at 10 years in federal prison.
You would be much better off buying exactly what you want, up front.
The receiver extension cannot have any holes in it (6 position) to mount a regular stock. It needs to be a specific pistol extension only, that can mount a slide on kak pistol "stabilizer".
Why .300 blk? Expensive ammo, won't get much use. Get a 5.56 unless you have a .30 cal supressor.
And dont possess that short barrel upper before you buy the pistol lower. You want to be in possession of the lower before you take possession of the upper, especially considering you already have other AR rifles.