Now that I have the SCR, after playing around with BCGs for a couple days. Besides the length, major difference between AR and SCR carrier is added metal to the rear, lower portion around the strut pin hole, presumably to reinforce the area. There is no hole for the firing pin retainer, this is accomplished by the strut pin.
See 3 options, hoping for some gunsmith/machinist input...
1. Add metal and mill strut pin area to replicate SCR
2. Drill strut pin hole to match SCR (without added metal)
3. Drill strut pin hole slightly above SCR location. There is approx. 1/16” space between the strut pin and firing pin. Will change strut angle very slightly, not sure of effect on action.