OK, this is a 1986 G-body. Notice that right at the rear of the door frame there is no cross structure on the floor pan. There is the rear front seat support toward the tunnel, but that's it. This is the least supported part of the structure. The ONLY thing that adds strength here is the rockers(hence why rusty rockers on these cars equals dangerous and death to the car). The front frame rails end right about where the front front seat braces are....about the middle of the door opening.
As for roll bars...the Autopower one that is in my '91 Daytona bolts to the floor directly above the rear jack points. The bolts under the car have thick washers on them too. That bar does not have the forward members like a 6-point, but it does have the rear members, a shoulder harness cross bar, and a diagonal cross bar.
Here's an A-body. Notice that where typically the door opening would be, there is a B-pillar instead. This will significantly increase torsional ridigity and overall body strength.
I'll see if I can get some P-body pictures, but they are going to look THE SAME!!!