A HG-1000 should be a Tecumseh compressor. The York will also work fine as they were both used on Mustangs and Cougars 67-73.
Converting to R134A with an original unit like the ones Ford used in 67-68 is pretty straight forward.
Recommended steps to converting your Ford R12 air conditioning system to 134a.
Systems w/o Suction Valve
Minimum Requirements
- Change or restore the filter-drier.
- Drain the compressor oil and recharge with 134a compatible oil. (PAG or Ester Oil)
- Insure that the system is clean and free of contaminants (Depending on component condition, this may be a simple matter of blowing out with compressed air or liquid flushing with an air-conditioning flushing agent)
- Replace or restore all rubber hoses with barrier hose.
- Install 134a charge port adapters.
- Charge with 134a between 70-80% of the original R12 charge amount.
- Replace any o-rings with 134a compatible.
Keep in mind with your original condenser you are limiting the effectiveness of your AC system. R134A runs a higher heat load on the condenser and that can equate to warmer air temps coming out of the vents.
Ideally you would want to replace the original condenser with a modern version such as a parallel flow. Here at Classic Auto Air we have reproduced the 67-68, 69-70, and 71-73 Mustang and Cougar condensers in a modern parallel flow design yet retaining the original condenser mounting locations and stock drier mounts. I have this condenser on my 70 Cougar.
This is a picture of my rad. support with our parallel flow version of the 69-70 condenser.