If youre adamant about not droppign ther tank, heres what you do...
1) cut a piece of rubber hose to fit over the drill bit to act as a stop, for instance if the bit is 3" long, cut a 2.5" piece of hose and slip it over the bit so the bit can only go .5" thru the metal.
2) weld a nut over the hole so that the bolt to hold it down has somethign to thread into, since obviously if the tank is still up, theres no way youre getting your hand in there to tighten it.
Problem solved, but dropping the tank is the best way IMO.
Also, i run my + wire thru the interior until the firewall personally, less chance of it gettin damaged by road debris, moving components, etc.