The key to a better fit for those qtr extensions is to section them by using a cutting wheel, bandsaw or whatever u have, just at the area where the extension begins to deviate from your body line of your car. As you have shown in your photo and stated, you had to drill holes and line the extension the best that it would fit, now using masking tape, lay a strip to mark your horizontal cutting line at the place where these extensions begin deviating from your quarter panel, while the piece is mounted per your photo, then remove the entire qtr extension from your car and make the cut. Yes I know they are expensive as I have bought three sets myself including the center sections, but unless you really can't use a cutting device, they can't be damaged beyond repair. The idea of sectioning the extension is to make them fit to the body as good as possible, and then fill in the gap(s) with fiberglas cloth and resin, overlaying the cloth as much as necessary to build up the integrity. These plastic extensions will not take a lot of the heat that is generated by the activation and curing of the fiberglas resin, so do small strips at a time and repeat after a cooled down cure. After making your cut, reinstall both of the cut pieces to your qtr panel to see your gap needing repair and then begin filling the sectioned gap with cloth and resin on the front and back of the of the extension. Be sure to match the lower extension piece curve to the bottom of your qrt panel...you might need to drill a temporary mounting hole and/or use small sheet metal screws to hold the lower piece in place as you begin the cloth and resin process. After the pieces become one again, i would remove the extension again and place a strip of cloth and resin inside the extension from top to bottom over the mounting ear and redrilled the mounting holes. (Not on the side that meets the qtr panel) I also used resin and cloth in the inside of the bodyline peak because these pieces aren't very strong structurally. After all of that, place the piece back on the car, bolt it down and then begin sanding the fiberglas down to match the quarter panel. You can use a little plastic filler ( bondo) if you need to and be sure to use a good filler primer to finish it off. This isn't a one hour fix, so take your time with the resin so that you do not melt your extension, thereby creating more work for you. I wish I had taken photos of this process before, but I don't have any at this time. My 80 Mirada is at my body shop right now and I'm gonna do this same procedure to my car, and take photos...its about 25% prepped for painting right now, but a 65 Fastback Mustang restoration is keeping me from it at the moment...but, I've done the quarter extension sectioning 2 times before and it works...just be patient.
