First thing to do is check the gas charge, there will be a sight glass and if it still has R12 just rev the engine to around 1,500rpm with a/c on max. Look and see if the sight glass clears of bubbles, if it does you have the correct gas charge. If you only see bubbles it will need to be leak tested and recharged, If you have R134A they have a blue and red adapter port covers. That needs to charged by weight 80% of an R12 charge, (see what they look like below they screw on to the old R12 ports and can be angle or straight).
The air should be cold even with R134A, while the early C4's do not have much airflow it will get nice and cold.
Check your heater valve shuts off, if you have hot water flowing through the heater continually it will affect the air temperature even though air is not going thru the heater core.
Some on this forum have found seals failed around the a/c evaporator allowing some air to bypass the evaporator coil.
The a/c is not as effective as modern cars with thermal expansion valves, this can be noticed by getting in a stinking hot vette that has been sitting in the sun. It will take time to get the a/c bringing the interior temp down, especially in stop start traffic.
The vette a/c works best when on the highway, you can quickly have goosebumps appear on your arms even with 100F-110F ambient temperatures.
Thanks i have r13a