I don't believe this will work together. With a 7" receiver extension (AR15, DPMS based 308AR), you need a 2.5" buffer. With the 3.25 buffer you will need the 7 5/8 receiver extension (Armalite AR10). As in this thread (link below), your bolt will not go all the way back. @98Z5V can you check my thinking on this? I'm not sure if the Sprinco orange spring will change this.
I would hate for you to waste a 3 hour trip. The easy way to test it is to hand cycle it when you put it together.