I think the factory cars only had torque boxes so I would think that subframe connectors would be stiffer than at least that. But I agree with @toolmanmike with just springing for both subframe connectors and torque boxes, stiffer is better imo
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.
