I did this about 15 years ago but had to use the bottom of the small window car. I cut the roof of the big window below the window opening and through the windshield posts. Would have been much easier if I could have used the complete back and if I had a wire feed at the time. Still worked out pretty good. Just remember to brace the part on the frame so it will stay in place. Hardest part for me was getting started. Just take plenty of measurements and don't get in a hurry. Good luck.