On our '66 Newport factory a/c car, there were no operational issues with the factory system, when using R-12, back then. I suspect that only underhood items have been upgraded with the Gen III motor? With the extra pusher fan for the condenser, road speed should not really be an issue, I suspect.
One thing of note, in the summer, almost every service station which did a/c work also had a large fan whose airflow could be directed to the car's radiator area to ensure the a/c pressures (in the hot service bay) were more similar to what they were when driving on the highway at speed.
Which gets back to R-134a vs R-12. As to the total amount of gas in the charge. The old rule of thumb of charges, when converting to R-134a, was to start at 80% of the R-12 charge and work up to more gas, which usually ended at 90% of the R-12 charge weight for optimal cooling. Starting at 80% and adding gas slowly until the cooling started to decrease slightly, then stop there. R-134a is more sensitive to over-charging than R-12 was.
I'm suspecting you are using the Gen III engine's compressor cycling switch?
One issue can be that you are using an 8CID compressor to cool what a former 10CID compressor used to. A higher rotational speed can compensate for this, though.
All of the a'c ducts slide together loosely. In earlier times, some might have had a soft foam sealing them, which is now long gone. Re-sealing all of those duct junctions might help with air flow volume, I suspect.
Even when our '66 Newport Town Sedan was "a used car", how the a/c outlets were aimed helped with rear seat comfort! Usually, the center ducts were aimed to just clear the top of the driver's seat, angled slightly toward the rear outboard passengers, but not enough to bother the driver and rh front seat passenger. This might take a bit to perfect, though. Then use the side vents to help the front seat passengers get ventilation. Everybody wants to have cold air blowing in their face when they get in, but this will also result in lesser-effective total a/c cooling performance, by observation. Vent aiming can be key in good total system performance, even with 40 degree F vent outlet temps (40 degrees F was the standard, back then).
IF the system might be "freezing up", air flow will be diminished and when the evap core thaws out, the water can be smelled in the vent outlet air flow.
Side issue -- other than the jute padding under the carpets and sometimes above the headliner, the older cars were NOT nearly as well-insulated as modern vehicles are. Any insulation was more toward sound insulation rather than heat (including the cowl insulation). Can it be made better? Certainly, just might mess with the OEM production looks and such, but hopefully everything will be hidden and make for a quieter ride.
Just some thoughts,
CBODY67
