Putting factory A/C on a car not originally equipped is a much bigger job than it seems. You have to have all(!) the parts, many of which are quite difficult to find at this late date—Classic Auto Air won't have them—and you have to do a great deal of cutting, drilling, and modifying. I did it on my '65 Canadian Valiant and because I'm a slow learner I did it again on my '89 D100. In both cases I had all factory parts and the results in both cases were not good enough to justify the effort and expense. If you'd like to add A/C to your '66, there are much more cost-effective ways of doing it.
But if only factory A/C will satisfy you, you will need:
-Complete heater/AC box assembly (lives up behind the dash) and duct assembly (bolted to underside of dash). Direct interchange is '65-'66 Dart, Barracuda and Valiant with factory A/C; a '67-'68 unit can be made to fit with some adaptation, and a '69-'72 unit with some additional adaptation
-5-button vacuum switch, 3-speed fan switch, temperature slider, bracket. '65-'66 Valiant, Dart, and Barracuda with factory A/C; '67-'72 with adaptation
-Dash bezel for control. '66 Valiant with factory A/C only, no other model/year interchanges.
-All underhood components. Compressor, condenser, brackets, hoses, pulleys, filter-dryer, expansion valve, heater water valve. You've got some leeway here; you can adapt aftermarket and/or later-model parts unless you need it to look exactly like it came from the factory with A/C.
-Upgraded radiator and radiator fan, fan shroud
-Cable-type accelerator pedal, cable, throttle linkage bracket, auto trans kickdown linkage, and compatible carburetor. The non-A/C slant-6 '66 A-bodies like yours used a rotating-rod throttle linkage for which there is no room when you install the A/C-heat box behind the dash; you have to swap to the cable-type setup used on '65-'66 slant-6 A/C A-bodies (also '64-'66 V8 A-bodies) if you want an original appearance, or else adapt and use a later-model "hanging" accelerator pedal assembly.
-Tinted glass all around, or else the A/C will labour fruitlessly against the sun's much greater power to turn your metal box on wheels into a solar oven
-LOTS of time, patience, tools, and money