I agree with 95geo. Oh, and as a side note, a gallon of gas is just over 6#. http://wiki.answers.com/Q/Which_is_h...er_or_gasoline
My vote would be to have a strap of thick rubber underneath the tank running from tab to tab. You could then bolt down through the tabs and I think it would be pretty secure. But, having it suspended while bouncing is not so good.