If it's like my '91, the high side port is right before the lines go through the firewall to the evaporator box, so you are measuring the pressure downstream of the condenser. It's only on cars that have the high side port before the condenser that you could see the pressure rise from a blocked cond...

It sounds like there is a blockage in the condenser. No parts of the high side should be colder than ambient.

If there is liquid meeting a restriction, the parts after the restriction will get cold because the pressure drop works like an expansion valve.

Back in the day, the recommendation for conversions was ester oil which I think is only available in 100 viscosity.

If it has the Interdynamic aluminum conversion fittings don't try to re-use those buy a pair of steel ones like the site sponsor sells or they may still be sold at NAPA.

Take all the fittings apart and look. GM Tech makes a very good point about looking in the stock location for front air only, the condenser outlet-- there must not be one there or anywhere else before the tee to the back. Some new condensers come with the tube already installed and it is up to the m...

If it has an accumulator that means it was originally a simple orifice tube system. Somewhere between the tee in the liquid line and the front evaporator there must be an OT. Factory dual systems were commonly OT in the front and TXV for the rear.

Now that you have the system open, check the OT as well. If there is black gunk everywhere you could consider a total replacement.

Do these have compressor lockout coded into the PCM like many other GM's did? If the compressor starts cycling rapidly (due to low refrigerant) the PCM will shut it down until reset.

If you're using Harbor Freight gauges there is a very good chance they are not coupling to the car properly and opening shrader the valves so you can read pressure. Your "troubleshooting" seems to have been very unscientific (example, "compressor didn't turn, so I replaced it". D...

Don't guess, find the leak. One that large should be detectable with soapy water.

Pressures seem about right. How cold is the low side line getting? Check that the heater water valve on the firewall is pushed all the way closed with the dash dial on cold. You could pinch off the heater hoses to be sure.