I believe even the '91-92 had R12 originally but I might be wrong. It is less about the gas the car was designed for an more about the sizing and the system used.
With the exception of the compressor going from a York piston type to a rotary, not much changed on the system since it was a dealer-installed option in 1975. It was undersized then.
In short, you need a '91 or newer with the silver, GM looking accumulator near the firewall. This system is also, from what I read, extremely sensitive to freon charge quantity so the only way to have it "topped up" accurately is to have it evacuated and filled by weight.
Onkel Udo
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