Why your 1968 GT350 / automatic probably didnt have a Holley

[KR500]

Improved combustion, its basically the pcv system

Gpd Im still working on finding that, but its the standard production part

IMCO has nothing to do with the PCV system. For the 1968 MY it consists of a different distributor advance curve and  leaner carburetor jetting.
Rodney

[KR Convertible]

So, the GT350 used an Autolite 4300 and the (early) GT500 used an Autolite 4100?

[Coralsnake]

These are true statements