I've seen that Stag spring first-hand, and it's a very stiff bastard. Put in an Armalite EA-1095 spring or a Sprinco Red spring, and see how it does - those two springs will perform the same, so just grab what's in stock somewhere. For that EA-1095 spring, that's the real Armalite part number, so so buy anything else that you can't comfirm is the actual, real Armalite part. Well, besides that Sprinco Red.
Everything else is where it needs to be for your recoil system. Buffer should be 5.200" long and 5.4 oz. Receiver extension internal depth should be 9 11/16" deep, from the back of it inside the tube, out to the top edge.