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I believe you mean unleaded gas. As far as requiring a rebuild, millions of cars built prior to the changeover to unleaded gas motored on just fine. Were there problems...yes, but I believe many were engines not maintained or simply high mileage to begin with and the owners blamed on unleaded gas.
If the engine is original and untouched internally, after all these years it needs a rebuild, I wouldn't blame it on the unleaded fuels. Remember it's 40+ years old and the vast majority of those years were after unleaded gas was mandated.