How much chassis stiffening needed?

A56 said
Not quite sure what you mean by factory cars, but my 340 cuda has all four of them (torque boxes). Agree that torque boxes and "heavy" torsion bars (1.03 or bigger) as well.

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.