Here's a thread for you guys from an a/c site I belong to. It shows the thought behind setting up a POA system a/c system on 134a on a 70 Nova which has the same a/c system as the Chevelle. It also shows how to test and calibrate the POA valve on the car. I've attached a video of that process.
It's well written and may offer a few pointers and insight that will help when you do your a/c system restore. https://www.autoacforum.com/viewtopic.php?f=3&t=3096
Below is a pdf file of the same thread. The a/c forum is slow and hard to access at times. It's almost like their server is on dial up. The last pdf covers flushing the system.
When you install your parallel flow condenser, you want to take full advantage of the airflow offered by your cooling system. It should not be mounted tightly against the radiator as this will add heat to the condenser. Seal up any gaps between the radiator and condenser to force the flow through the condenser. The more airflow you have through the condenser, the more efficient your a/c system will be.
Listen for the pop sound when he pressurized the system...pop test, means poa is functioning. Fro there it can be adjusted. When you install the poa and TXV, pour a little refrig oil through them and then allow them to drain. This helps them open and function when you charge the system. If you want to store poa, run the oil through them and store in a ziplock. As long as there is oil coating the insides, it will last for a long storage period. POA's out of systems that have been open for extended periods will deteriorate and need to be rebuilt and calibrated. When switching from r12 to 134a, the valve also needs recalibration as seen in the video.