Whenever I would remove a fuel pump sending unit I would put the rubber O-ring in the freezer while I upgraded the pump to a walbro. The ring if left out would expand for some reason and not fit later. The freezer would keep that from happening, and sometimes get a little smaller but it would expand back to normal size fairly quick once it's out. So if you do freeze them, install them asap.
Although from the date on your last post you should be done. How did it go?