If I were to say blow air in the overflow hose and plug the hole under the gas cap lid with my finger, I should not be able to blow any air through that hose right? Or is there another point of entry for fuel overflow inside the tank somewhere?

 

im pretty sure there are two tubes that go through the tank, but they aren't connected - they're two separate tubes - so if you blew in one end and plugged the other - no it would/should block it

 

Ah ok, I'm draining the tank now. I heard a gurgling when I blew in the overflow hose and plugged the top, and it would leak a bit when I filled it past 3/4. Must have a crack in it somewhere. I have a plan to just cut the broken part of the pipe out and clamp some hose in there to bridge the gap, if it's accessible.
Lots of people plug the overflow hose when it leaks, but that seems like a good way to get water in your tank.