Generally not a good idea to put screws/bolts through fiberglass. You could have used a fiberglass bandage to secure the bracket to the inside of the fuse, just epoxied the bracket in place, or, best yet, used Hysol to secure it. In cases where it is necessary to bolt through the fiberglass, I epoxy an oversize piece of ply or G10 in place on the inside before drilling.
In the case of your bracket, any side-to-side motion in the bracket induced by engine vibration is likely going to cause more issues. I would reconsider whether the bracket is really necessary. A good 30-minute epoxy, assuming you roughed up the inside of the fuse with sandpaper and then cleaned it thoroughly with alcohol, should hold the tank platform just fine.