Rear gas tank install question

[Ozzy2013]

I have been all over the sight and internet looking for an answer i put the LMC rear mount gas tank in .I have the filler neck installed through the bed .The sending unit had the gas line that goes to the engine .It also had a second fitting on the topside of the tank that i guess could be used as a return line for fuel injected .Mine is carb so do i plug this fitting or can it be used as a vent?Thanks Oz You can see it next to the frame rail upside down of course!Buying a camera as soon as i can afford one the truck is killing me in parts !