I just installed one in a 70 Nova and it was tricky getting the seats back in after...but a little twisting and maneuvering and it eventually dropped in. I got the seat back to drop down where it then contacted the bar and I marked it with some white chalk around the rear leg of the roll bar. I then slid the seat bottom back in and it actually dropped into the lower locks without needing any cuts at all. I pulled them back out and cut the lower front cross wire along the front of the seat back and made a couple incisions into the seat vinyl to give it some relief and dropped it back it. It fit around the bar and did so without any stress on the seat or fabric and fits great around the bar.
Not sure how much different a Chevelle is from a Nova but found this very doable. Just had to be patient while twisting the seats back in from the passenger side. The seat back goes in right side up and left side down low and needs to be twisted while the face slides over the bar from the passenger side. Be sure to watch the lower tab doesn't tear into your headliner and that the left side doesn't scratch the lower door jamb as it slides in from the jamb. Hope this helps you out.
