[70SBC10BOB]

Internal vacuum leak would be caused by a bad gasket seal on the intake. Instead of leaking on the outside where you could locate it with carb spray along the intake/head joint, it would leak inside or at the bottom of the same joint. The leak would be into the engine valley creating vacuum in the crankcase. Sometimes this type of leak will cause blue exhaust smoke because oil is sucked in.

Keep swingin' at it.