... On the top surface of the tank is the fuel pump sender 'plate'. it has a rubber gasket that can go bad, or get out of position. You can see this if you lift the back seat cushion and unscrew the access plate. ...
Though unlikely compared to all else mentioned, fuel line can leak from that same location as well.
If it doesn't leak when the tank is below 3/4 full, then it's less likely to be that gasket.
From personal experience, any replacement gasket has to be a *perfect* fit like the original. I know there are some new replacements being sold through parts stores that aren't a perfect fit, and will leak when you fill it up at the station.
If you replace this gasket, take a plastic handpump that's approved for gasoline with you on your first fill-up at the fuel station. That way you can suck fuel back out of the tank incase that seal leaks. You can leave more than enough in there to get back home.