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I found not only timing affects vac but AFR definitely do as well. Get closed to the correct AFR and vac increases.

just had a look at a log from my 304, it idles at 800rpm with 40kpa this is a stock cam but the motor has ported heads and slight increase in compression and im pleased to say there is no sludge or shit in mine as its only been together and run a few times

OK it seems my engine is still working as it should. I was concerned because of info on the problem with those engines enjoying a feed of cam lobe like the earlier six cylinder Holden's. I did cook the engine big time about 5 years ago. Drove it till it stopped and would not crank over till it cooled down. Can only imagine what the piston skirts look like but I have always run Mobil1 oil so that might have saved a few internal components.

did the same thing to my v6 engine. seized it, but was ok after it cooled down. Done good 50-60k since then but its got more leak down and lower comp as a result (easy to turn by hand, makes less power and more fuel usage), though vac seem the same?

My engine has little blowby and no unusual noises and still goes like a rocket so I think I am very lucky but I am sure if I pulled the engine apart I will see some skirt pickup etc from the overheat.

I plan to build a new engine this year for it. The old one will go for an autopsy as its been thrashed hard more than 2/3 of it mileage. So I'd be interested to see whats worn/not worn.