It varies from year to year and from model to model but there is usually a good sized hose that comes off of the filler neck somewhere and attaches to the sending unit. It is just a big breather hose that allows the air in the tank to vacate as fast as you are filling it with fuel. When the tank gets full all the way you actually get liquid fuel in this line and it is at this point that it is possible for it to start barfing fuel out of it. Really there are only a couple of connections on that tank that are capable of leaking larger amounts of fuel and only do so when the tank is full. If the supply, return, or small vent lines were compromised they would leak all the time or in the instance of the little vent line they would leak but not large amounts like it sounds like you are loosing. Certain tanks also had a fitting on the top corner by the driveshaft that was capped off, if that developed a leak it would fit your criteria. I suggest that you roll under it with a flash light and a long telescoping mirror and have a look see.
I know the later 760's with the MLS rear ends had a pigtail filler bent hose and it was the vent hose, about 5/8", that would split and barf fuel when the tank was full. Your only other option is that it is the sending unit itself leaking around the O ring like Tony said. It is unlikely but by no means impossible.

On a different note, have you given any thought to the fact that two half tanks of gas cost the same as one whole tank?

Mark