Well, if it is a typical fiberglass 27 body, it is hollow from firewall to the very back of the body. When you build them you have to add plywood and fiberglass material to glass in bulkheads and floors.
I have a 27 glass body on my one car, and I installed the floor, backrest, and sides in wood, and glassed that all into one piece. But the back part (called the Turtledeck) just hangs there in space. My body bolts to the floor via 6 bolts that are only in the area where you sit.
I didn't cut doors in mine for the very reason you outlined........it severely weakens them. Some people have glassed in door jams and cut doors successfully, but I have seen too many that don't work out, so I eliminated that problem.
Here are a couple of pictures that might give you a better idea of what I am talking about:
Don
See the backrest made of glassed over wood? Behind that is hollow all the way to the back where the tail lights go.
Here, this shows the inner liner of wood and glass pretty well.