Overlap on the manual 1-2 upshift is why rear servo mods are so important when other performance mods are made, quicker release of the rear servo eases the overlap but it can never be totally eliminated.

In earlier units the overlap was less noticeable because there was a delay between moving the shift lever/button and the actual shift; in '70 the factory changed the profile of the 1-2 shift valve to force an upshift but this exacerbated the overlap.