That small hose is the vent from the tank up to the filler. Sounds like you removed the hose, and plugged the nipple at the tank, and so it now has no vent. You might need a vented gas cap. Or redo the stock/original hose vent set up.
Actually The '68 cuda does not have any nipples on the tank. The small hose vents from the top of the filler neck down into the left frame rail.
Oddly enough just this past Friday morning I was thinking about how the tank in my car vented since I had replaced it with a new one in SS. I went and dug out the original tank and it did not have a nipple. Disaster averted!
