Your motor is connected to the battery. The Black wire is connected to the negative terminal. So you can connect the tank ground to the negative terminlal or any place that is also connected to the negative terminal.
Usually the fuel fill and vent have a ground wire too.

As for corrosion of the tank, make sure you seal the hatch with silicone so water doesn't get in. If you remove the foam & tank you'll see that it is in an Undrained compartment. Any salt water that gets in stays there forever. Thats what corrodes tanks.

yes they do have the original wire still connected.