Is this gas cap vented

Look up the part no. A 70 / later "pressure vacuum" cap is not technically vented. It is pressure release. I'm against using a vented cap on these girls. I believe, that at some point, you will have fuel all over the fender.

One option is as above, duplicate the 69/ earlier system

Another is to either leave the 1/4 vent tube in place, and run it "up high" so it won't siphon. Plumb it into the air filter bonnet if possible