I would ignore most of this thread. A bodies did not ever use what we think of as a "vented" cap. None evap controlled cars up through somewhere around 71ish (might have been a split year) were vented with a non vented cap and a tube welded into the top of the filler neck. Look up a 69--70--71 shop manual over at MyMopar and read it. The evap controlled cars CHANGED almost every year. There is a "famous" photo running around this board of the changes in what are called "pressure vacuum" caps over the years. There is a BUNCH of them. Pressure vacuum caps are NOT VENTED in the usual sense. They are designed simply to relieve excess pressure or vacuum in the tank. THE VENTING on evap controlled cars is through the evap control mechanism, and the key feature is the 1/4" tube going from the tank to the engine bay. THIS IS the fuel tank vent.
If you want to circumvent this (eliminate it) I would look up the 69/ earlier vent system and duplicate it
