Slant won't drive over 25mph without cutting out

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"?