Gas tank question HELP!!!

[painterljp]

Have a 72 gmc longhorn, well Sierra Grande. Has three tanks. Has the selector on dash for am assuming just the gauge reading. The selector on floor next to seat with no markings. The valve under truck has 4 ports. One from each tank then one going out. How is this gonna work with the return line on the pump? The fuel pump has 3 ports. One from tank one to carb and one to ?? Had a 2 line on it when we got it. Have just rebuilt the engine and don't really want to mess with other two tanks for now. They need pulled and cleaned, but really want to start New engine and just run it off tank behind seat for now. Guessing other is return? Sending unit have to replace in tank behind seat. It is 2 line. A friend here has a new one gonna go get today. But unsure if return is going to cause fuel to be returned to what tank? It is a four valve switch under truck. Any info would be appreciated. Thanks in advance.

And is this factory, Aftermarket?