If the A/C system is fully charged with R12 and other parts are clean and proper, there should be no reason it can't chill you out of the cabin, even on a humid 90 degree day. Those systems were more than capable if the condenser is clean, and the correct refrigerant is used. I get 38 degree air out of the ducts on my 1977 T/A during a hot day.
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1976 LeMans B09 Freeway Enforcer, 455/M40 Smokey
1977 Trans Am, 400/M21 Black/Gold Bandit. 44K actual miles
2017 Sierra SLT 1500 Z71 4X4 
2019 Canyon SLT Crew 4X4 
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