Don't know your temps.. .you need pressures and temps to know.

Anyway, if the low and high side pressures are high- your system is overcharged. The 86-89? cars had only 15 cc compressor where later ones are 17 cc's. I wouldn't use R134a, I'd use either R12 or an R12 subsitute for better performance. That being said- you should be fine to about 100F where things really break down.
The idle performance can be dreadful with a converted car.

What is the condition of you FAN CLUTCH????? Is there leaves or part of the raditor plugged with bugs?? I'd bet the new fan/fan clutch would make the world of difference. Less stress on your headgasket by keeping the engine temp rock-solid @85F. The pressures could easily be not enough airflow over the condensor. Did they run a water hose over the condensor when checking pressures???????

Michael