Most of the aftermarket "vented" caps only vent one way and do not vent excess positive pressure. They only vent when there is negative pressure (the fuel pump drawing fuel). If you take it out of the cool garage and go out when its hot it will still have pressure on it when you open the cap. This was responsible for blowing out my sending unit gasket.

Now I run a vented cap with a 1/16" hole drilled in it (as Bjorn so wisely showed us). I couldn't find a vented cap that vents both positive and negative pressure. A '9 has an open vent right in the filler tube and vents up by the trunk with a small hose. I figure I'm doing the same thing, but could possibly spill fuel on a hard launch (which I almost never do. LOL !!) Haven't spilled any yet that I know of.

I cannot comment on actual stock '68 FGF vented caps because I can't find one.