There were two types of Mopar units offered in ‘65.
The dealer installed unit consisted of a self-contained evaporator/blower unit (“knee-knocker”) not unlike what every other company offered, except that it was quite long and intruded into the passenger foot space a great deal. It was designed for the big cars because a/c was expensive and not likely to be installed in the less expensive small cars. It was called Mopar Cool Aire and can be seen in the original owners manual or shop manual.
The factory unit integrated with the heater and kept the “summer door” on the passenger side. The air was expressed through a slim chrome register centered to and bolted to the underside of the dash. Like the dealer installed units, your body tended to freeze toward the interior of the car and cook toward the exterior of the car, at least in hot climates. There was no register available at either end of the dash to correct this problem.
The underhood components were identical for both dealer installed and factory units. A Mopar compressor only was used, and no other brand. The Mopar compressor tended to bog down the slant six and noisy belt “flop” was a problem fixed by adding tensioners in the 70's.
The ‘63 and the first half year ‘64 factory a/c units were identical to the dealer installed a/c units. The firewall was changed in January ‘64 to accommodate the new small block V8's distributor. The firewall was also changed to accommodate the new factory a/c.
You can easily retrofit the ‘64/12 factory a/c unit to a 64 1/2 to '66 non-a/c car. It requires drilling a few holes in the firewall for the unit attachment studs and the a/c lines. The control unit in the dash is changed from the lever system to the vacuum controlled push buttons. I’ve done this myself without difficulty.
I purchased a Classic Air “perfect fit” system designed for the ‘67 to ‘72 firewall and installed it in my ‘63 Dart GT convertible. I would not do this again, but would instead look at one of the new compact evaporator units on the market. While the firewalls are close, they were just different enough to make installation very difficult. It’s not Classic Air’s fault. They did warn me.
I personally don’t know whether the ‘64/12 to ‘66 firewall is identical enough to the ‘67 to ‘72 firewall for a/c purposes. If it is, then installation should not be a problem.
Another approach would be to install a “knee-knocker” that doesn’t intrude into the passenger compartment as much. The advantage of this approach is that it keeps the original heater and passenger side 'summer door".
Hope this helps.
If you do a search under my name, you'll see that I've posted before on this topic. My installation travails can be found here:
http://www.forabodiesonly.com/mopar/showthread.php?t=2772
–Walt Jackson