If you prefer the coupe look of the original GTO, are located in a climate that favors hardtop cars, or you just don't like convertibles, I can supply the kit tailored so that you can install it without removing the roof. I have a line scribed in the mold where I terminate the layup for this purpose. With this termination, the rear tub drops down over the Datsun roof and rests on the rear quarters. Slight trimming is required for best fit, but it is much easier to trim to exact fit than have to add material.
You will have to do a little hand fabrication around the rear of the hatch opening, and around the quarter windows. The original GTO did not have quarter windows, and many of the early GTO replica kits incorporated a vertical extension of the tub to cover the Datsun window. Many builders chose instead to remove the quarter window and fabricate a metal panel to be welded in place. My preference (and the reason Datsun designed the quarter window) is to leave it open for visibility. It's a small area, but makes a big difference. Without it, I feel claustrophobic.
If, like me, you want to retain the quarter window, you will have to do a little bit of hand fabrication with foam and fiberglass to create the radius from the top of the quarter panel area into the quarter window well. I have a handful of builders who have done this, and can supply photos and additional information for clarification if need be.