A lot of the performance had to do with track conditions, like altitude and barometric pressure. Take a stock 383 RR and run it in Minnesota in the heat of the summer, and you are looking at 94 to 96 mph at best. DA would be somewhere around 4500 ft! Take the same car and run it on the east coast or down south in the fall, with a DA of 0 or mineshaft conditions, and guess what? It would pick up tremendously. I had a bone stock 69 440 RT charger, best was 14.92 at 96 mph. More than likely due to a DA north of 3000 ft. The Magazine cars got special treatments like the 440 exhaust (2 1/2 Vs 2 1/4) a re curved distributor, re jetted, so I know for sure there were ringers out there as well. All in all, they carried thier own wieght with the performance crowd very well. Simple little things like removing the spare, airing up the tires, a tweak of the left front torsion bar, a cold thermostat, proper cooldown and you could run another 1/2 tenths quicker than the average Joe out there as well.
What RPM did you shift at? Was the car cooled down well?