How deep is my Hull

[stul]

After fixing brackets to the floor of my rib.
Making a new stainless steel petrol tank to fit under the seat(two seater consol with steering wheel)at the moment it is fixed down with some 6mm pins sticking upwards with nuts and washers on & angle brackets.Tried undoing these nuts and pins result pins come out ,but not sure how they where fixed in the (plastic bottem) are they drilled and taped into bottem?,how deep can i drill ?
Any advice would be helpfull,thanks
Stul
4metre Narwhal Rib

[stul]

rawlplugs
are the answer