Blazer tank vent

[MNorby]

I just put in a new poly tank in my 71 k5 and am in the middle of plumbing everything in. I removed all the old nasty lines that went around the back of the frame to that vent deal inside the cab and basicall have my feed and return line on the pass side and my fill and fill vent line on the drivers side. I am thinking I need a basic tank vent though so the tank doesn't colapse or expand. Doesn't look like the cap is a vented gas cap. There is an extra port on the poly tank I could run a vent somewhere and I still have that tube in my cab for the original vent could I just run a line from that extra port to that in-cab vent deal? Deos this make sense to anyone? Thanks