Paxton option in 66? Yeah...right.....
Many kits might have been installed by dealers, but it was an aftermarket accessory. Paxton offered a lot of kits for different applications. Most likely Paxton supplied the editors of Motor Trend with a kit and maybe even help installing in exchange for a favorable mention in a road test. Product placement plain and simple.
I think Paxton has offered kits through Ford dealers for mustangs and such over the years.