Which one is the fuel return?
 

Got a 2000 Silverado, originally a 4.8L. Put a 6.2 in, doing custom fuel system, removed all the factory fuel lines and swapped a Walbro 255 pump in. Used factory fuel sending unit, but didn't pay attention to which line was which when I took it all apart. The main feed line is obvious, being the largest, and I think the return is the other outer line, assuming the middle line is the evap line. Am I right or is the center line the return?

I'm talking about the ports coming right off the fuel sending unit on top of the tank.

Re: Which one is the fuel return?
 

You're going to have to look on the bottom of the fuel pump/sender module hat to see what's what.
Return should have a pipe and vapor should have some kind of screened hat... I would think.
Your other option would be a walk in the u-pullit graveyards to get a look.