Jeff - my FJ60 developed a water leak around the fuel filler neck and had about 1 gallon of water sloshing around in that cavity. I removed the tiny rubber drain plug at the bottom and replaced the gasket around the filler neck.
I thought if if you removed the cargo panel on the inside, you could see part of the back side of your repair area. Maybe I have forgotten - I sold my 60 years ago.
Colo Springs, CO