mpgmike said

Check slop in your timing chain. That would cause low vacuum. Many /6's had those plastic covered cam gears. The plastic gets brittle and breaks off, causing cam timing issues. A worn cam lobe would hurt vacuum as well. Since they removed the zinc oxide from engine oils, more older engines are suffering from worn cam lobes. You didn't mention what year this is, but a clogged exhaust (like a cat) would also kill upper rev HP and give you low vacuum readings.

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rustycowll69 said

I know you said you were only running 0.040" spark plug gap. Stock is 0.035". Normally that number shouldn't cause misfire if everything else is up to snuff. By increasing the spark plug gap, you are increasing the resistance for the spark to jump from one electrode to the other. But electricity is lazy and likes to travel the path of least resistance, so if there is another easier path it will find it.
Sometimes low vacuum is a function of too lean a mixture. Which could be from a vacuum leak. I would try a leak down test, if the leak is not otherwise obvious like a leaky power brake hose or vacuum advance hose.
Can you be more specific on the idle vacuum reading? What number is it? does the needle fluctuate? Fluctuate in a consistent, regular pattern? Does the needle "float"?

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