Stock tank question

[jcd9730]

Considering an LS swap, have some questions on the fuel tank. I want to keep the stock tank. I have read that a lot of guys weld a bung on the tank for a return, but their fuel sending unit only has one line. Mine has two (I'm assuming one is return one), with a vent line at the top passenger side. Can I just use the return line on the sending unit, or is this not actually a return and something with emissions?

For the vent line, do I leave that as is or close it off?