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You say you set the ignition timing by vacuum gauge. 18" of vacuum is about right. The y block generally likes 10-12 degrees of advance at idle with the vacuum disconnected from the canister.
Due to the rings of the harmonic balancer having a desire to slip the timing marks may not be correct. A vacuum gauge is a very accurate way to set the timing. The method I use is to have the engine idling at normal idle speed and twist the distributor to where I get the highest steady reading and then twist the distributor back to where the vacuum drops by 1" on the gauge. You may have to reset the idle speed if it revs too high as the mechanical advance will start to cut in.
Take it for a test drive just to make sure it is not advanced and pinging.
Regards