Plugging Timed vacuum advance hose

Mattax said

But your cars are no longer stock. So your two setups, especially the 340 with the XE274 cam may not act as AJ described in this statement "Obviously, as soon as the throttle is opened, the vacuum falls and, with it, the low-speed vacuum-assisted timing."
The manifold vacuum from idle (at say 700 - 800 rpm) to off- idle may go up. That would largely due to the valve overlap causing pretty low vacuum at 800.

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So far I agree with every thing you have said, especially this part. This characteristic can be very advantageous to know if you have a manual trans. If you set your can just right, you can really pump up the part-throttle take off, as the vacuum continues to rise, up to about 2200 rpm (with typical street cams) as the overlap period stops pumping air back into the intake. This can make a marked improvement in low rpm driveability.
Automatics, not so much,lol.
My examples were with stock to very mild cams, which the OP seems to have.