The small vent tube line off the sending unit.
When I did the LS swap I changed the tank and sending unit to 87 and up style for fuel injection with an electric pump in the tank. Put a wilbro 250 in an had issuses when we where trying to get it started and it wasn't getting fuel. The guy thought th pump was bad and had an aeromax 350 or something. Not sure if it has to do with a higher volume pump or something else. I have the vent hose run up by the fuel filler to get it to the highest point I can back there.