None of the following is to be taken as "Gospel", just what I have sort of figured out over the last 10 years or so.
1. The Allied Armament trigger is of the original TNW format. Possibly still available from them, but probably more than the value of your first born child.
2. Allied bought these triggers from the folks in Texas that copied the TNW pattern, I think they go by Military Gun Supply.
3. ALL the TNW and MGS/Allied triggers, operate in a mucho "Variable" area of movement in relationship to the back plate/lockframe as opposed to the more common KMP/Lee Tool/Daves 100 etc. trigger designs. So additional clearance is definitely required(milling of the cross bar and the backplate slot) to allow the triggers to move "Higher" than is usual.
4. EVERY gun I've seen/built etc. has had the "Relief Cut" done in the rear of the barrel extension for the front of the trigger bar.
5. It would follow that the lockframe cross pivot member would need to be clearanced as well, since as I stated above the TNW/Allied Triggers are known for the "Wide Variation" in the arc of movement required to release/reset the sear.
6. The steel "Box" you mention was originally counterbored for a coil type trigger return spring that pushed against the trigger pivot cross member, but I think that was discontinued as the trigger pull was horrendous.
7. Without casting any disparity upon the TNW/Allied trigger design, they work fine just a PIA to work with, should you ever need/decide to replace the trigger/sear, by all means contact Tom at KMP and get one of his units, all things considered, you'll be light years ahead of the game by doing so.
From the land of the "Texas Cowboy Militia"