My amp is behind my seat, but it is grounded in the big 16mm bolt BEHIND the passenger tail light. I sanded it down, then grounded it. If you dont sand down you wont get a good ground and your system will crackle at higher volumes.
just pick a stock ground...as mentioned any metal as long as u sand down the paint...sanding is important and not drilling into ur tank is also equally imp. u can use a circuit safe logic probe 2 test for ground. avoid using self tappers if u can