I have ran both the AR 3911 and the NGK 7173 both extended tip only change was plug in my case car ran same et but with the AR 3911 I gained 1 mph and the car idles much better this was back to back test same day 30 minutes apart.

70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4