I would not advise using the hull
sides as an integral wall of a fuel tank
. It can be done, but, should it spring a leak or if you get fuel
seepage, you will never get rid of the smell and stink without ripping everything out and redoing it.
I have seen fiberglass
integral fuel tanks
FAIL on PRODUCTION BOATS only a couple years old. There is a multi-million dollar 130 foot mega yacht out that has a permanent smell of diesel down below, due to this very problem.
If it were something besides fuel, it might be ok.
It would be better to construct a fiberglass
tank the shape of the bilge and bolt/block it in.