A unibody car uses the rear "c" pillar of the roof as part of the structural strength of the chassis. All of the cars we choose to modify are blessed with strong 4" to 8" subframe chassis rails under the floorpan that takes a huge part of the load on the car.  When you cut the rear roof "c" pillar extension that goes to the trunk of these cars(whether for a factory convertible like the Audi A4 or the VW mk4) the chassis is reinforced at the factory with stronger add on panels to accommodate for the loss of strength that the roof "c" pillar provides.  The VW/Audi cars as well as the Charger are very tough as cars so they make great foundations for a conversion.