I don't have a sun machine but was told the slot was intentionally manufactured wider because of the 3030 Camshaft needs this extra advance at idle to run smooth and allow a better transitional response from Vacuum to Mechanical advance which happens the micro-second the gas pedal is pressed and vacuum goes to zero your on mechanical advance.
The slot doesn't affect idle advance - that's a function of the vacuum advance can; the slot only defines the upper limit of the centrifugal advance system. Corvettes with the "30-30" cam (1964-65 L-76 and L-84) used the "236 -16" VAC (same as the current VC-1810/B28) and full manifold vacuum on the can, so the VAC was fully-deployed against the stop at 8" Hg. vacuum vs. the 9"-10" Hg. they produced at 900 rpm idle.
Not sure about the '67 Z/28, but the '68 and '69 with the 4053 carb used a "ported" vacuum source on the side of the carb baseplate for the vacuum advance, which provided ZERO vacuum to the distributor at idle; this was done intentionally to retard the timing at idle to increase the exhaust gas temperature to make the A.I.R. system afterburn more efficient with only 4* BTDC of initial timing and no vacuum advance until the throttle plate was opened.
This is easily cured by capping the "ported" source nipple and teeing the distributor line into the rubber hose from the angled full manifold vacuum source nipple on the front of the baseplate to the choke pull--off diaphragm; then you have full manifold vacuum on the advance can at idle, adding 16* of advance to the initial idle setting.
