I see you have never worked on a automatic climate system in a old Caddy. it has a lot of electric over vacuum and vacuum over electric controls to move the doors which all have rubber seals to shut off or mix the set temperature. and vary few people around that will rebuild the electrical components. most of vacuum controls are few and far between NOS stuff or used. with them having 3 different types of systems between 63 and 76. systems that do not share the same components. you can rebuild the system but it can be a expensive proposition. all the factory one does is cool the front the rear has a separate unit to cool it.
still if it was me in that boat I would call these people
http://www.oldairproducts.com/catalog/?ibp-adgroup=ppc&gclid=COu2mpPVjMECFedzMgodCzgA_w
tell them what you are looking for they can set you up with a compressor that will fit into your brackets and a heating and cooling system that will go in your car. tucked out of the way that will operate threw you current ac duct work. defroster ducts also. then all the man that puts it in will have to do is remove your existing system putting the compressor and condenser in place designed for your car and a small amount of playing around to get the duct work hooked up and the hole in your fire wall covered. what you spend on the kitt will save labor putting it in. it is a reticulation type system with only the controls hanging under the dash.