Yeah, there have been a number of stories and magazine articles over the years that touched on the various manufacturers prepping their test cars "above and beyond" what a legit showroom unit could run. Super Stock & Drag Illustrated even ran a follow up story about the Ronnie Sox A12 440 6-BBL Road Runner test that became the basis for Plymouth's ad campaign.

The great conditions at Cecil County the day of the test weren't enough to account for the 4-5 MPH advantage that particular car had over the other A12 drag tests of the time, which SS&DI believed were due to it being a pre-production car with higher compression and a slightly hotter cam that had been considered for production before the build specs were finalized. Oh, the carbs had been stolen off the car before the test and the replacements didn't come from the parts division, they came from the engineering department. Read into that what you will.

FWIW, the follow up article was written by Ro McGonigel, one of the two SS&DI staff who participated in the testing with Ronnie Sox. He's a way more believable source than some of these kinds of stories have.

EDIT: Not sure where the rest of the story can be found, but here's the part that talks about what I mentioned above (and in more detail). LINK