Retarding will reduce detonation and allow you to run a lower octane. If you want to run regular, just fill it up with regular and then advance it until it begins to detonate/hard start and retard it a hair from there. When a modern vehicle with engine management's knock sensor picks up detonation (could be from low octane gas) it retards the timing to reduce detonation. Same theory applies here, if you run a lower octane, retard your timing. You will not by any stretch of the imagination be able to set up your timing with premium to the point that it's almost detonating, then expect to run 87 fine. Modern vehicles can adapt to that, these old mammoths can not.
I've also noticed... if you get the timing right on the detonation borderline and the gas stations switch from winter grade fuel (typically pre memorial day) to summer grade (different oxygenates- aka additives) you will notice a slight performance drop and possibly the need to tweak timing in order to get her back up to snuff.
