I mounted an under-dash in my 1964 Valiant and 1965 Dart. It was finicky to fit them so they weren't "knee-knockers" and the hoses kept clear of the accel pedal. I used one which came in my Dart (mounted it more competently) and bought a vintage one on ebay for my Valiant (might have swapped them). Both are squat and not very wide. I didn't just hang them from the lower dash like gomer shops did, instead running a strap from the brake pedal support or the top of the dash. In one, I used a U rod to go around the glovebox liner (used failed coping-saw). I also ran a lower strap to a screw in the front of the trans tunnel to secure it under the dash better. Retrofit shops and dealers used to run hoses thru the firewall all the way to the compressor and filter/drier. Instead, I used O-ring fittings at the firewall. Made my own hoses, using a MasterCool crimper and ferrules. The vintage boxes use flare fittings, so had to source proper mates on ebay. In my 273, both hoses pass thru on R side, just under the heat-only OE climate box. In my slant, the small hose runs thru there and the bigger return hose passes high on the L side, routing above the brake pedal bracket. Differences mainly because the connections are on different sides in the AC boxes.
Perhaps as overkill, I tapped a wire inside the box which is hot whenever the box is on. I ran that to the engine bay via unused bulkhead terminal to trigger a vacuum switch which closes a water-valve in the heater hose (1 vac line, spring return). That disables the heater core when AC is in-use. Simpler to tap the AC clutch wire, but then the water valve would only shut when clutch was on. Overkill since the heater box has a separate blower, so even if the heater core is hot, not much hot air would come out. Might even be bad if you wanted to run the windshield defrost with AC running.
A disadvantage of the replacement climate boxes is that they have separate controls so your dash heater cables would no longer be active and just for looks. For some reason that bothered me, but more an emotional problem than tech.
