My 3/8" sender has a 1/4" vent tube. You can get them without. If your vent truly is 5/16", let me know. I use the vent for EFI return and it is a bit small for that.
The tube on the fuel filter neck is probably 1/4" or smaller. That is an atmospheric vent. It loops up high in the trunk then down thru the rubber filler tube grommet to underneath. Check that the end isn't plugged with undercoating or gunk. If it doesn't vent, your tank can draw a vaccum and starve your fuel pump as you drive if you have a non-vented cap, or even collapse your tank. I expect that later emissions cars w/ charcoal cannisters don't have that vent. You didn't tell us what you have.