A packaged system is great, though the drawback of having to adapt your dash for the heater controls. Hopefully all the mounting and belts work, which may not match all intake manifolds and accessories. An AC-only under-dash "knee-knocker" box keeps your heater box and its dash controls. A bit clumsy to then have two cabin blowers and boxes, but you can locate so not too intrusive. Dealer retrofits (Chrysler "AirTemp") used knee-knockers. I did AC on my 1964 and 65 A's and 65 C-body.
My 65 Newport (383) had an aftermarket knee-knocker with York compressor which was fine, so I just redid the AC hoses and put in a better aluminum condenser (cheap ebay). The hoses had run past the throttle pedal thru hacked firewall holes w/ grommets, which was clumsy and sometimes hit your foot. I ran the new hoses thru the passenger side, w/ firewall fittings just below the factory heater box. I added a binary pressure switch to the clutch wiring (relay coil side). I also popped open the AC box and tapped a wire which is hot with switch ON (not just the clutch output). I ran that wire to a vacuum switch in the engine bay which actuates a water valve (generic $12 ebay). That blocks the heater water when AC is active. Not really needed since a separate blower, but wasn't hard and did it while replacing the heater hoses and added a section of metal tubing on the inner fender (ebay for some car, w/ a little bending), which secures the heater hoses and is actually cheaper per ft. I used silicone hose. The Newport has a wide cabin and the knee-knocker is long and thin so doesn't bother the passenger after I rehung it better than the shop did (Sears, Western Auto?).
My 65 Dart (273) had a Tecumseh compressor, which is York-ish w/ same bolt pattern but cast-iron with a larger pulley. So heavy that it had cracked the aftermarket bracket. I sold that beast on cl to a guy to repurpose as off-road air-compressor (youtubes). While one can buy Sanden compressor brackets, I welded up my own, using arched-slot bracket parts I had. They came on a used Sanden (Chevy?). Anyway, the slots are the hard part, so I just needed to weld a base plate and arms to engine bolts. My 273 also may not fit aftermarket brackets since has different intake bolts (size and angle) plus my changes (later alum water pump and p.s. pump). I fit the Sanden slightly above the timing cover.
My 64 Valiant convertible (225) had no AC. Since manual steering, I used the 3 bolts at the p.s. pad on the block for custom Sanden brackets. I used the "universal" (cut and weld-your-own) set, which are 2 plates with arched slots. I used thick structural brace from Home Depot for the base, with its end bolting to the L engine bracket and the front having an arm to a water pump bolt. Hard to get Sanden brackets for a slant (some listed, but vapor-ware?). A long post gives AC options for a slant and I show my brackets there. I pulled factory Sanden brackets from a 1978 Volare, which includes p.s. mount for a Saginaw pump. I could change to those if I go to power steering someday, though they come close to the battery in my narrower engine bay.
I had one knee-knocker from the Dart and bought another on ebay, (Chrysler Airtemp label). I painted both to look purty. Like the Newport, I ran the hoses thru passenger side firewall fittings, exc on the Valiant I ran the return hose thru driver side firewall above the brake bracket since the compressor is on L side of engine. The Dart had the same hose-hack by the driver's firewall pedal. As the Newport, I added a binary P-switch and water-valve w/ metal tubing, and a new aluminum evaporator. I added a fan in front of the condenser, using one for my 1985 M-B on all 3 w/ various relays (Hayden on 2, for my 1996 Plymouth on other).
Now is the time to buy AC parts cheap on ebay. I've bought new Sanden compressors for $50 from liquidators (shipping accidents and such). Indeed, I have a few extras stashed, plus a new York. I paid ~$70 for the AirTemp knee-knocker, or you can buy new Chinese ones for ~$120. I bought the MasterCool ferrule crimper, which I've also used to make oil cooler hose for my 1984 & 85 M-B plus re-hose their AC. When done, I can resell it for close to what I paid. I bought AC hose and ferrules on ebay, so have parts to make more hoses. My M-B uses custom fittings, but by cutting off the ferrules I re-uses the hose ends. For new stuff, get parts with O-ring fittings (#10 comp inlet, #8 h.p. line, #6 liquid line). I got a new evaporator for the 1964 (Honda Civic?) but had to finagle to match its "peanut fittings" (later Chrysler used too, not standard). Use only "barrier" AC hose (Viton lining?). I used "reduced barrier" for some where a smaller-size ferrule fit (M-B has thick #12 suction hose). I use HC refrigerant (Duracool) w/ PAO 68 oil in all my vehicles, even the 2002 Chrysler after replacing the compressor. It doesn't absorb moisture and works slightly better than R-12 Freon.
