You don't need to flush etc. Have someone recover and recharge by weight. If the low side still doesn't come down and start cooling, the control valve in the compressor is likely the problem. Diagnosis of a variable system has to start with knowing the charge is correct by weight.

More likely it's refrigerant from a Chinese factory, imported in container sized tanks then canned in the USA. If it has a reputable brand name on it they should have checked it for purity. It's a little better imported in bulk then canned in the USA because they can't cheat by putting real 134a in ...

Do you remember what the pressures were when running? R-134a will dissolve internal parts of very old driers which were not made for it. A 1969 drier is likely to be one of those that will blow its desiccant out when 134a goes through it. This will clog up the TXV and other parts. So yes the thing t...

Is the high side port before or after the condenser?

Yes you should remove the stock shraders when putting on converters that include their own, but the converter shrader valves have a special tip on the bottom end to push down the stock shrader if it is still installed.

It only takes a couple oz of 134a to put 50 psi in a system for a basic static pressure test. You want to keep the pressure below the saturation point (i.e. no liquid R-134a in the system) so there will be an immediate pressure drop if there is a leak.

It doesn't hurt to keep dye in the system all the time to help find any leaks that might occur.

Charge by weight first. It is very hard to diagnose a variable compressor system without first ruling out the possibility of over or under charge. The pressures don't respond to the charge amount the same as a conventional system would. And always test at 1500 rpm. A lot of systems barely work at id...

The old-style condenser isn't helping you any. What is the high side doing? A lot of conversions end up undercharged to mask poor condenser performance.

The control valve doesn't do anything as long as the low side is above the valve's set pressure-- the compressor operates at full capacity. Once the set pressure is reached, the valve makes the compressor de-stroke to prevent the low side from going lower. This keeps the evaporator from becoming too...

Cleaning the condenser would certainly help. If there are a lot of bent fins from the larger bugs and small rocks, consider replacing it. But you can run what you have it's not too unusual to be pushing 350 in triple digit heat.