The AC kits were often installed by the dealer, but were otherwise
the same as on the assembly line. They even came with the hoses
"precharged' with freon.

That's all absolutely ridiculous.  Aftermarket add-on air conditioners were
probably more common than factory integrated units then.  Even the early 64
1/2 Mustangs had hang-on units from the factory.  What insurance company is
telling you that an add-on ac makes it modified?  I think in judging an
incorrect ac could cost you authenticity points, but certainly wouldn't push
you into modified.  J.C. Taylor has never asked anything like that on any of
my cars.  By their thinking my '64 Daytona Wagonaire (which I have owned
from new) would be modified because I replaced the all-white steering wheel
with a red and white one.  I also installed wider wheels and put and
electronic ignition system on it- more modified?
Reminds me of 1965, in Hawaii, when I bought a hang-on tachometer to put on
my Wagonaire the salesman told me I needed a state modification permit to
install it.  Guess who installed it anyhow with no such permit.
And the vintage tag?  Again- ridiculous.  Around here the only thing that
determines the antique plates/use of vintage plates is the age of the car
(and of course they can be used for daily drivers).  My son's somewhat
modified '79 Corvette has antique plates on it as does his '72 Satellite
Sebring with Cragar mags.
Paul Johnson