Likely because your car has the incorrect fuel pump for its application. Or someone has bypassed the fuel return line, if your car is/was so equipped.

If your pump looks similar to this, and has the 2nd port plugged, it is designed for a car with a return line, which for some reason is not being utilized. http://www.summitracing.com/parts/CRT-M4690/?rtype=10

If your pump does not have a fuel return line and you don't intend to replace it, then it is better to simply run a pump that does not have the extra port, eliminating the need for hillbilly plugging and the extra potential for a leak. They are widely available, similar to this one: http://www.summitracing.com/parts/CRT-M4868/?rtype=10

Keep in mind that whether or not you have a fuel return line depends on whether or not your sending unit on your fuel tank can accept one. You claim to have a 400, and most of the 400's indeed came factory equipped with it. So if yours was so-equipped and now no longer has it, I'd be curious why it was plugged and/or removed.