There should be a rectangular cover between the helm and rear seats, that is held down by a bunch of screws that will cover the gas tank. You may need to remove the rear seats first if you have a bench seat.
https://www.dropbox.com/s/lfs7ioudwrnkn ... boat2a.jpgSee it in this pic? Most older F/W boats have this hatch. The anti siphon is screwed into the fuel pick up and it will have the smaller fuel line connected to it. Your will see a large one that is the tank fill. On mine these are both at the rear of the tank, the vent is up at the front of the tank.
If yours doesn't look like this post a pic of what you have.