Josh beat me to it. Sit in your car, centered in the seat, put your belt on and get it locked into place and move your body around trying to hit the A pillar and B pillar... going to have a VERY difficult time pulling that off, or else sell the car it is a death trap. You will typically be able to bust the door window out with your head in a side impact but that should be about it.
Add a cage that is 2-3" closer than those items between gaps and tubing size itself, make it ROUND not flat like a B pillar so you can really crush an area of your skull - do the same thing... won't be hitting the A pillar, the cross bar across the top of the windshield (the hoop)... maybe but probably not, but the B pillar area and the bar immediately behind your head hoop, probably so.
I built the cage in my car custom to tuck everything as tight as possible, moved the seat inboard almost 2" to center up on the wheel (and rotated the steering column to match) and I can't hit the bars with my head and have 1" of clearance in general. WITH the padding, I can just touch the front of the hoop with my thick, top air helmet, so in a big crash I will make contact with the thick SFI padding but not worried.
Keep in mind SA rated helmets v. M rated helmets have the additional testing for hitting roll bars, because the initial contact is a point (sphere on cylinder) and only becomes line contact as the helmet deflects, so force is distributed over very small areas. If that is your skull... hope you enjoy eating your meats and starches with a straw.
So, my point (I am long winded)...
you can do a cage, but relocate your seat and make damn sure you have at least 1" (IMO) of clearance straining against the belts in any direction. I would make sure that is including padding - don't ever "expect" your head hit anything, period - do all you can to avoid contact with your noggin. That SFI padding will kill you just as quickly with no helmet - it only serves to increase the area of contact for your helmet, the force is still brutal.