Gas Tank Vent

You must have a 74/ later car?

What makes you think you have a vented cap. NO A body every used a truly "vented" cap. The later ones were "pressure vacuum"

Using a vented cap only invites fuel slopped all over the side of the car.

Without a diagram of what you did to the vent lines, no way of knowing. If you leave the tank hooked to the trunk gear "as advertised" then run an open tube from the original line which had gone up front, and ran that "up high" and back down, that should be OK.

The BEST way in my opinion, is to braze/ weld a vent line into the filler tube exactly like the 67-69 and some 70 cars did.