A roll bar by itself will add very little stiffness. Torque boxes AND subframme connectors are both doing the same task. IMO, taking into account your restrictions, forget the torque boxes and engineer in say a welded massive 4" Vertical by 3" horizontal .083wall frame connector attached to the most optimum points, and call it a day.

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.