My 73 Challenger was a non A/C car. I used a Hot Rod Air system. It was their Universal system for Mopars. The compressor fit perfectly. They provided all the mounting hardware that is necessary. I pulled the heater box from under the dash, fabbed up a plate for the firewall to cover the old blower motor hole and drilled the heater and A/C holes into that plate. I bolted it up to the firewall using the original bolt holes (I did have to drill one extra though). I opted for a kit with underdash vents since I didn't want to cutup my dash, though given the questionable originality of my car I should have just gone with the in dash kit, it gives it a cleaner look.
The blower and heater core are all self contained in their own box that mounts up underneath the dash in the stock locations. Being a universal kit it didn't have the correct brackets for connecting it in the stock mounting locations, but it does come with brackets for mounting your own. I still need to wire up the controls (got side tracked by having to fix up the floor pans and transmission tunnel, if you've seen my other posts you know what I'm talking about... safety first before comfort). and get the system charged, but overall it was an afternoon of work and not much hacking up the car.
I think Classic Auto Air has a system made for the E-bodies that fits in the stock location and also have the option of indash or underdash. I went with Hot Rod Air because they came in a couple of hundred cheaper. One note, if you go with Hot Rod Air, make sure you get the bulkhead fittings for the heater hoses, the kit only comes with some for the A/C lines. I ended up snaking the heater hoses through the holes in the fabbed bulkhead plate and using grommets to protect them.
