Need some quick help with a fuel pump issue

[AirSpeed]

Trying to replace my fuel pump in my 72 GMC 4X4 with 350/350. I ordered a new one and it has one inlet and two outlets, the one I'm replacing had only two. I just looked under my truck and found a third line running from right under the battery, mounted to the frame then to the gas tank. The end under the battery, mounted to the frame has what's left of a rubber line stuck to it. Is this the return line or is it supposed to go to a charcoal canister? I'm thinking the pump on my truck now is the wrong one and I should reconnect the the unused return line and use the new pump. Is there a reason not to do this?