Remove the headliner? Sorry, I couldn't resist.
I am assuming you are talking about the CE C3039 8 point bar kit?
If so, you are concerned about welding the rear struts to the top of the bar?
If it were me, I would do one of the following.
1. See if the main hoop and the rear struts will tilt forward as a unit. Is so, tack the main hoop to the reinforcing plates on the floor. Fit the rear struts and tack them to the main hoop making sure they are in good contact with their mounting points in the rear of the car. Some welders paper will protect the headliner from stray sparks. Cut the tacks at the floor of the main hoop and tilt forward. Finish weld the struts to the main hoop. You should still protect the headliner from spatter.
2. If tilting is not an option, I would drill holes with a hole saw which will allow the main hoop to drop through the floor. The steps are basically the same as above but now when you cut the tacks on the main hoop to the reinforcing plates you will have to slide the reinforcing plates out and let the main hoop drop down.
Plan ahead and do a lot of dry runs.