Face pulls and seated dumbell cleans are a staple finishing exercise for both Westside upper body days. You should probably do 1 of these for 2-3 weeks and then switch.
If traps are REALLY your single most critical weak point (and it is possible as they both make your bench and squat setup more solid) then I would do them most training days, or even as an extra 10 minute workout on "off" days.
Advanced westside in my opinion involves three main things:
1) Focus on pushing you DE loads ever faster. Don't increase the raw weight unless you are blowing up 8 x 2 squat and 8 x 3 bench in about a 10 minute time frame to start the workout.
2) Figure out your one weak point in each lift. For the squat it is usually: Glute/hams, lower back and abs or upper back in my opinion. For glute hams, focus on arched back GMS, for lower back, bent over GMS and for upper back safety bar squats for max effort. For bench I break it down into A) Off the chest: Focus on lats and traps; Midrange: focus on shoulders-but primarily on scapular rotation-shoulder push from the scapula, and lockout: closegip boards.
3) Picking 1 or at most 2 areas in need of hypertrophy-probably in line with your weak point, and give them extra rep work.
However, I would use bands early on as well. If you are really moving fast in the squat, you'll need them to keep from throwing the weight off your back.