I looked through the 1968 factory shop manual for you. Unfortunately, for 1968 there is really no mention at all about the fuel tank and or the engine fuel venting system. Generally, the AC cars had a return line with a restriction in the 1/4" line near the fuel pump. The restriction in the line was quite small. If memory serves around .040-.045" hole There is usually only a small dribble of fuel in the vent line. Not absolutely sure but I think 68 would have a vented gas cap, essentially open to atmosphere. The carb float bowl is vented as well as an extra vent lever operated to be open at idle. Sorry, but that is all I could find .
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