Will the body I already have fit the GTX?

If your body was designed to mount on a VW pan that was shortened 14 ½”, and matches the basic physical dimensions of our Nostalgia or Lancer then with some internal fiberglass removal, your body should be able to be mounted on the GTX frame.  The critical dimension is the height of the body at the rear of the passenger compartment.  This is the area where the struts are mounted, and cannot be changed.  Your body needs to be tall enough to clear the rails in that area.  To confirm that dimension, sit your bare body on a level floor.  Lay a long straight piece across the top of your body at the rear of the rear jump seat.  Measure vertically, from the floor to the BOTTOM of the straight piece.  The minimum height can be no less than 23 ¼”.