If I was to ever put a cage into my XJ, it would be something oriented to the A pillar, with the B pillar, being secondary (but also necessary to a slightly lesser degree).
Most every XJ, rollover I´ve seen, the roof collapsed in the area of the windshield, back to the B pillar. Windshield pillar, seems to be the weakest area.
My second job, was night tow truck driver. Seen a few XJ rollovers.
Also seen more than a few people, crack there skulls, on the roll cage. Street racers (and off-roaders), with a cage tend to push it just a little too far, on occasion. False security?
Have also seen the seat belt, mounts, come out of the body, with a piece of floorboard still attached. Or the seatbelts, tear, after being weakened by the son (UV) and ozone. Or brained by a tool box. Sometimes the cage is kind of academic.