[Kenth]

Retarded ignition timing advance (no vacuum advance at idle) was implemented 1968 to heat up the cylinder heads to reduce emissions.
Most 1967 and earlier used full manifold vacuum to the ignition vacuum advance for highest efficiency and cooling at idle speeds.
While driving at normal speeds there is NO difference between used source for the vacuum advance (load compensator).
Also, there is NO way to compensate lack of vacuum advance by the centrifugal advance alone.
Most Pontiac vacuum advance units starts at 6-8 in/hg and adds max 20° at 15-17 in/hg.
I have found any 400 engine with a 068 or smaller cam, or 455 with a 744 or smaller cam can use full manifold vacuum to the vacuum advance.
With hotter camshafts a ported source will be beneficial.
And, with a properly adjusted/modified carburetor idle/lowspeed circuit there is seldom if ever necessary to alter the factory ignition curve.

FWIW