I had a similar issue with my '57 fire truck years ago (322 Nail Head)...I lost all power going up a mountain and had to nurse it to the top riding on the shoulder. Ended up getting towed home. After days of head scratching I accidently touched the 3/8 main vacuum hose that stabbed into the intake and saw it move. Turns out it was just sitting on the pipe nipple and would float up and down causing a big leak randomly.
I also had burnt exhaust valves with only 20k miles on the engine, so vacuum leak and burnt valves were my issues. Fixed both issues and drove it another 20k miles before I sold it.
Mike B
