My first attempt to convert our 1988 W124 wagon (black w/black tint windows) from R12 to R134a, wasn't very successfull.
We only changed out receiver/dryer; expansion valve; and pressure switch, drained oil, and refilled 80% with R134a and the proper oil. On a 95-F trip to Las Vegas it cooled down to about 78-80 F.
This time, we evacuated everything, replaced the single pusher AC fan with the dual AC fans from a 1990+ W124 R134a vehicle, as well as the R134a specific condenser.
Also, replaced the viscous fan with a 16-inch SPAL puller fan at 2370 CFM.
Our trip to Las Vegas in 95-F cooled the cabin to a very cool 65-F.
In short, get more airflow with electric fans, and a high-flow/high-density condenser.