Low side equalizing to the high side. completely normal

The EPA was planning to abolish r134a. A recent court ruling has decided to stop this from happening. Same thing for r410, R404. Thank you President Trump.

The above is exactly correct. Spray a water hose on your condenser. If all pressures drop, you have proven what was said. Use R12 and be done with it.

You can access the low side just about anywhere. Be advised, R134a in Corvettes has not been the best choice of refrigerant.

Why not just use POE Bava oil?

If the circuit is the same as a 69 Dart, there is a temperature control switch. It is in the circuit right before the low pressure switch.

Overcharged? Airflow across the condenser? Non condensables Was the compressor new or rebuilt? What was the clutch gap on replacement comprtessor? OIl in system and how much?

I did not read where it was suggested to add stop leak. Dye is "colored oil". Check your evaporator carefully for leaks. A good place is to check the drain hose with a sniffer. Also, place a shower cap around the front of the compressor and place the sniffer into the shower cap after sitti...

I have some R12 in small cans that also contains dye.

one thing stuck out of your plans. You were going to vacuum the system to see if it holds and then open it. Do not open a system while under a vacuum as it will draw moisture into the system. Instead, put some refrigerant or better yet, N2 if you have it then open the system.