Well,looking through my old GM manuals shows that PCV systems were available for Chevy and GMC 6's in trucks back in the late 1940's.Looks like the PCV piping was from the valve cover to the intake manifold where the "PCV" was located.The manuals mention if a PCV system is retro fitted,it may be necessary to increase the main jet by one number,no mention of idle mixture issues.
With a proper operating PCV system there should be a slight vacuum where outside air is taken into the engine.A bad valve cover gasket may not leak much oil,but air could be drawn in upsetting the carb mixture.
Mybe you can use a restricter in the vacuum hose or another type of PVC valve?
