Had the same problem. Found that the intake manifold was sucking air from the bottom of the gasket. The fix was to grind the manifold. Found that the Fenton hedders were holding the intake at a slight angle and the intake would not fit the head flush. Grinding took place on the bottom of the intake for clearence between it and the headders. Intake sets flush to head and vacuum leak went away.

1953 Chevolet 5 window