I put the DSE cage in my finished Firebird. I removed the front/rear seats, tacked weld the down bars to the main hoop, and was able to remove the whole unit outside the car. IIRC, I had to jack the car up slightly to gain some room to maneuver the cage out of the car.
You can see the process in post 87 of my build thread.
https://lateral-g.net/forums/show...t=43170&page=3