Agree the answer is no it couldn't be ordered. Never seen one with it with the claim nor documentation of it being done by a dealer. Only buyers I thing might have even considered power steering on an otherwise performance car would be someone that ordered an automatic 66 K code - but again I've not seen or heard of one in all these decades
May I remind you that a 66 GT K Convertible was used in a MCA judging seminar held in the late Bruce Cunningham's garage that had dealer installed power steering. (Bruce and Rich did most of the restoration, the car belonged to G. Adcock. The picture is from the 1998 Mustang brochure.)
And then there's 6S2377 with A/C. Not assembly line, but -
If it weren't for exception, we would not have any fun.
Jim