The Shelby fiberglass cars do not use the birdcage tubing that surrounds the cockpit and doors. The engineer felt it was not necessary because the fiberglass is rigid enough to support itself. On the aluminum cars all that tubing is needed to wrap the body over, and it needs to be accurately placed with all of it smoothly transitioning from piece to piece. Since the 'glass body is already molded into shape, the only reason I can see for installing any of it is to mount the door striker. Shelby gets around that by forming a one piece scuttle hoop in the rear:
All the superstructure tubing is 3/4". The early Shelby cars had the aluminum flashing riveted to the door, hood, and trunk frames just like an aluminum car. Then it was laminated to the skin. Now the use Lords adhesive to simply bond the tubing to the skin.