Just cut out the old tailpan and make all the edges on the car straight, then make a template out of posterboard, transfer it to some 20ga, and cut it out. Form the radius' over a suitably sized round tube (I've used everything from a welding tank to the light pole in the alley), then butt weld it into place and you'll end up with a seamless back end on the car. For strength, you would also want to bend the bottom 90 degrees to the inside, or leave the bottom edge long and wrap it around a piece of 1/4" round tubing.... Could even get creative and french in a license plate mount, or cutouts for you tailpipes...... Stock is boring anyway!!!!!