I share your curiosity relating to stiffness of body and chassis of the 116s. Working on Gleaming Beauty, say with either front or back wheel sections jacked up individually, one will experience a slight binding when opening doors during the process, indicating some twisting of the body. Under normal driving conditions, pottering around the suburbs, or 110 or 120 km/hrs. on a Freeway will not show up any handling issues. However, whilst I have never driven GB past 170 km/hr with no apparent handling issue, I would probably be less confident at 200 to 220 kms./hour.
Just as an aside, when comparing the 6.9 (my own anyway) rear suspension traction on loose or gravel surfaces with the Rover 3500S De Dion rear suspension, the set-up of the Rover is far more tractable and re-assuring. The De Dion set-up, used very much by Sports Cars of the '50s and 60s, is simply brilliant in putting the power down where it counts. Regards Styria