Slant won't drive over 25mph without cutting out

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.