[rotorwrench]

The Cabriolet cars have extensive wood framing that supports the sheet metal and doors. The sheet metal is just an outer skin. The wood frame was built to hold the body to be more rigid while the top was down. The main bow in a sport coupe has a lot to do with keeping the body rigid. If a person substitutes a Cabriolet style main bow on the Sport Coupe type body, it may need some more steel in the body to beef things up a bit more for structural rigidity.

The roadster body belt line sets lower for that reason plus it has extra gussets at the B pillar to hold the door latching more rigid. It has small light weight doors. With the body lowered down, it had more structural metal behind the seat to hold the quarter panels better. Now a coupe body with no top would require something more to hold the heavier doors for latching and to keep the quarter panels from moving around. All that stuff would have to be considered.