Fiberglass question

[JohnC]

Chopped strand is not a structural fiberglass. You would certainly make the box from 1708 or another structural configuration of glass. 1708 is preferred by lots of folks because it's strong and versatile, it does a lot of things well (like tank/coffin boxes).
I would recommend posting your project here so that some of the glass guru's can help you get a good plan in place.

If it were me I would plan on 2 or 3 layers of 1708 for a structural box that will support the weight of a full tank of gas. You would also need to consider how it is bonded to the stringers and where the mounting points are for the gas tank.

Is the tank aluminum or poly? How big? Are you planning to foam it in?

In most of the rebuild threads I have seen here the builders enclose the area for the tank between the stringers and install bulkheads fore and aft to make the shape of the box. You could also build the box and then drop it in. The bottom of the box has to support the weight so a sandwich cored bottom would be relatively light and strong (plywood or structural foam core).

There are others here that have done it. Post your ideas and you will get help. You came to the right place!