Sounds to me like maybe someone has changed the vacuum lines around in some way that you have manifold vacuum on the dist but the specs call for the vacuum to be ported (no vacuum at idle). If you have vacuum at the dist at idle, then unhook and plug the line then set the timing to 3 deg. Reconnect the vacuum and drive the car and see if it pings under load. If it does then retard the timing 3 deg making it TDC and drive again.
What market was the car built for, USA, Euro, World? This will make a difference. My 500 engine was made for the Mexican market (I think) and is a low compression engine and runs better on regular than it does on premium. I don't know what the official spec calls for but I run it at 7 deg btdc.

PaulM

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84 500 SEL (307,xxx miles)