The pressures sound a bit off, I expect lower on the low side and higher on the high side. That could be a thing with the refrigerant you are using, I'm not familiar with its p/t curve.

You can't see the evaporator through the center vents, the silver thing is the heater core.

Check to be certain the monovalve isn't leaking and letting hot water into the heater core. If it is, you are (partially) reheating the cooled air. You can run this test by using a pair of vice grips to temporarily clamp off a section of heater hose. Use something to protect the hose from damage - a slit section of rubber hose works well.

The climate system has no filters on this car.

One issue which decreases a/c performance on this car (and all others...) over time is the evaporator becomes coated with dust, dirt, filth, and grunge. It makes a quite effective insulating barrier which prevents heat transfer from the air stream to the evap.

If I had to guess, I'd say your compressor is a bit weak, based on the pressure readings. The system doesn't appear to be undercharged, because the low side reading is plenty high enough - too high, really.

- JimY