Yeah, as n9xvt said, the second, smaller line is the fuel return. If they disconnected it, that was probably a band-aid fix for another issue, possibly a weak fuel pump or a return fuel line that was leaking.
All of the Eagles with the 258 had the return fuel system. If you look closely, the steel return line will be buried somewhere on the driver side fender wall near the strut tower. I think it was only the 4 cylinders, or possibly only the GM Iron Duke motors that didn't use the return system. I'm not sure if it's a difference in design of the carburetors or some other reasoning behind it, but one thing I have read is that part of the return setup is that it keeps vapor bubbles from getting into the carb when the fuel becomes heated/boils within the fuel lines under the hood. That's why the return line/filter is supposed to be installed at the 12 o'clock position, so the bubbles will rise up to the return hose and not get pulled into the carb.
When I got my 84 dodge van somebody had already taken the return system out of the loop, basically they just cut the return hose and stuck a screw in the end. Really, everything on that van was repaired with hose clamps and sheet metal screws, it was a wonder that it ran. But that van ran terribly rich at idle when I got it, and I think that was why, the pressure had nowhere to go was getting forced into the carb. Although in theory, the fuel shouldn't have been able to be forced in unless the float needle wasn't seating properly. I tried putting the return line back on. There were 3 spots on the 15 foot long steel line that were irrigating my driveway with 87 octane, so I didn't get a chance to find out if it ran better that way. I just leaned it out enough at idle that it wasn't loading up at lights, and left it that way.