Clicking A/C on the option sheet added almost 20% to the price of a base model A-body in the 60's. Adding $330 for A/C on top of a $200 A/T option was a lot money back then.
Whoever ordered your 66 Dart decided to spend $330 for A/C and leave the $100 273 engine at the factory. My guess is people living in AZ, NM, TX or S Cal back then voted cool to be more fun than speed.
Factory A/C is rare on early A-bodies....I have a factory A/C, 4speed Formula S Barracuda that is without exception, my most favorite car ever...and I suspect is rare but how rare is anybody's guess since no records are available for that particular option.