How much chassis stiffening needed?

Some cars had all 4, some had rears only. I have yet to see factory information which cars had what combination other than convertibles were supposed to have them on all 4 corners. You want a solid car? Add all 4 and subframe connectors. Torsion bars and suspension mods go from there depending on how you are going to use the car.
Yep, appears this car is more street driven and that heavy front end on /6 torsion bars is a potential corner washout. Still "factory cars" definition?